Don't know the answer but I had a question--are you going to pipe the sides of the cake and if so, would the peanut butter stay put? I would be afraid it wouldn't stick and slide off, but it's a great idea.
Corn syrup sounds like it would work--give it a try. Or what about piping gel ??
Let us know how it works.
Serves/Yields: 3 cups. (16 servings)
Prep. Time: 15 Minutes
Cook Time:
Category: Frostings, Cakes with Nuts
Difficulty: easy
If you like peanut butter you'll love this frosting, great on chocolate cakes but for a real peanut butter experience try it on a peanut butter cake. If you don't have cream, you may substitute milk.
1/2 cup butter
1 cup creamy peanut butter
4 cups confectioners' sugar
1/3 cup cream
1 In a large bowl, beat butter and peanut butter until light and fluffy.
2 Slowly beat in 1/2 of the confectioner's sugar.
Mix in 1/4 cup of the cream.
Beat in the remaining confectioners' sugar.
If necessary, add a little more cream or milk until the frosting reaches a good spreading consistency.
Makes enough to frost one 2 layer 9 inch cake or one 9x13 inch cake.
Source: Suzanne Stull
Contributed by: on Monday, April 26. 2004 at 16:39:09
why not try this?
Then you don't have to worry
Jen
I'd definitely go with Jen's recipe rather than straight pb. I'd use natural creamy peanut butter for a "peanut-ier" taste.
Bec
On my space shuttle cake I piped a bead border with my BC with PB added to taste. If it seems thin when you add the PB add some more PS. You could also cut down the butter in your fav. recipe some and add the PB to the butter and cream it well first then add the sugar and milk or water if needed.... Good luck.
Here is a link to my cake.... the border is messy but that is cuz I am not good at those yet! 
It piped like regular BC....
http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=312453
I love this peanut butter cup filling and you can pipe with it as well.
http://www.cakecentral.com/cake_recipe-2316-Peant-Butter-Cup-Filling.html
I use a recipe very similar to Jen's and it works well both as a filling and as an icing. If it's not stiff enough for piping, add more powdered sugar.
Warning: if there are any leftovers, you will find yourself with spoon in hand until there's only the empty container!
i'm glad it turned out 
I use a can of regular cake icing from the store and add peanut butter it stands up real good theres a peanut butter cake in my photo's, I also mix peanut butter in a can of chocolate frosting use it on a chocolate cake and crumble reese cups on top, everyone loves this after adding peanut butter to your icing it stands up real well for piping. hope this helps
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