Sure you can use it on the crust. You just don't want to soak the cake into sogginess. You just want to use enough to add moisture, not actually soak it.
Assuming you're going to put anywhere from a thin (crumbcoat) to a regular layer of buttercream on your cake, the fondant will be unaffected by the syrup. 
tell me more about using soaking syrups on scratch cakes, please.
How do I make the syrup? How do I apply/use it? What does it do to the cake structure/texture? Can I use it just to soften a crust?
Click on this link http://www.cakecentral.com/cake-decorating-ftopict-10274-soaking.html+syrup
I gave a brief description on how to make and posted pictures to show...
is that teaspoons in your chart?
Edit to correct myself.
No, Actually since this chart was from the Spanish forum the "t" actually stands for cup. So if it says: 1 1/2 t it is for 1 1/2 cups. I am sorry, I didn't realize that she had put it that way.. 
Just recently I got brave and add cherry flavored syrup that my kids use on their snow cones with a red velvet cake. I like things premade... so that was sitting around. Wow what flavor. I was also thinking of adding the syrups they use in coffees these days. Non - alcohol is the way I need to go for my family.
I have literally changed the flavors of a basic cake yellow by soaking them with the syrup.
For example: when you make the syrup you can add any flavor extrac:
Lemon
Orange
Pineapple
etc...
I can't say enough about syrups, i find it very strange that they are not used a lot in the US, it's standard in French pastries and European cakes. I got to a point where i don't even measure my syrups i pour some water add corn syrup and any liqueur or flavoring or jus even and voila. I use a spray bottle to get an even soak. the good thing about syrups is that you can make a cake and add a complimentary flavor which translates to fancy and exotic. I made a butter cake and soaked it with caramel liqueur syrup but when i say it's a butter cake soaked in caramel liqueur people get impressed! it does add a lot to the cake.
Real bakers do still use the syrups. I know b/c the baker for my wedding cake drowned the cake in a pineapple syrup. It was so good, even the thick bottom layer was moist all the way thru 4 days after it was baked. It made a great morning-after breakfast for the family.
I can't say enough about syrups, i find it very strange that they are not used a lot in the US, it's standard in French pastries and European cakes. I got to a point where i don't even measure my syrups i pour some water add corn syrup and any liqueur or flavoring or jus even and voila. I use a spray bottle to get an even soak. the good thing about syrups is that you can make a cake and add a complimentary flavor which translates to fancy and exotic. I made a butter cake and soaked it with caramel liqueur syrup but when i say it's a butter cake soaked in caramel liqueur people get impressed! it does add a lot to the cake.
It is a standard practice in Puerto Rico as well. The people I have talked to there rarely do cake mix cakes... So, the clients are used to scratch cakes soaked in syrup.. They do it for birthdays as well as weddings... For the weddings they just use more exotic mixes for syrup like adding liquors.
Like you, I eyeball it when I am soaking the cake. I soak it to how moist I wanted or the customer. I had a lady that wanted "really" moist, so I added more syrup than usual. Others like less, so I adjust it. So, I don't follow a strict measurement to soak cakes.
The chart I posted however its good as a guide.
When do you put it on? When the cake is warm or before frosting or? I wanted to try this once but wasn't sure.
I've heard a couple of different answers.
The way I learned, by a professional baker which book I follow on all my recipes, is to soak the cake after it has been completely cooled. The syrup can be added when its warm or cold, but not when its hot. She said that it could ferment, especially if you are using flavors like pineapple and lemon.
So, you can wait for the cake to be completely cooled, if you are torting it, tort it, wet the first layer, fill it with whatever filling you chose, put the next layer on top and then wet just like the first.
After you have soaked a cake, don't try to pick up. It will break up because is now wet and not as firm.
Marialoves cakes
You and I find ourselves answering every single syrup question...looool, are you a soaker like me hehehehehehehe!!!!
Just as Marialoves cakes mentioned before, I follow the same steps. I m sorry Ive been doing it for so long I don't pay attention to the steps anymore.
I was shocked to see that the people in US don't soak their cakes...It is a standard procedure in Puerto Rico. Just to give you an idea, I couldn't sell one cake if it did not have the syrup in it.
People actually come to you and ask what flavor syrup will I use. They assume I'm using it and I actually always do because it is part of the basics in cake baking.
The only difference in my technique from the ones I've seen here is that I don't tort the cake at all. Here in PR we don't really tort cakes unless the client asks, we just soak and frost and decorate.
The only difference in my technique from the ones I've seen here is that I don't tort the cake at all. Here in PR we don't really tort cakes unless the client asks, we just soak and frost and decorate.
This is what I found too. I learned how to decorate cakes here in the US, but using Puerto Rican cake recipes. I knew how to catered to the people from Puerto Rico because I ate those kinds of cakes as well.
However, I did notice that there is no torting and filling for the cakes. I even have a lady that orders cakes from time to time and asked me not to fill the cake. Just to soak it in the syrup and ice it!
I have even gone to weddings and celebrations alike and they don't fill the cakes either.
You could make a thick syrup to add flavor, but I would not soak a cakemix cake. It would just get mushy.
I'm a child of the 70's and everyone poured flavored glazes over cakemix bundt cakes or poked holes in the cakes and poured glaze over them. This method will add flavor to a cakemix cake without ruining it.
I love soaked cakes for the moisture they add. My one suggestion is to use a denser cake so that it doesn't fall apart. You can mist the cake or, for a really moist texture, poke holes in it with a skewer or chopstick and pour the glaze or syrup over. I did this recently by pouring a rose-orange syrup over a fenugreek cake from the cookbook "Jerusalem" and it was unbelievable!
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