I would think the chocolate topping would thin the icing down too much 
I haven't heard of it either BUT it eems to me it would be fine if you keep two things in mind. The topping has sugar in it and the cocoa is usually unsweetened, may need to add a little extra slat or flavorings to cut down on the sweetness and the other thing is whatever liquid you use (milk, water etc) you would need to reduce that since the chcocolate topping is a liquid. If you try it, let us know!
I'd love to know, if you try using this, how it worked out, how it tastes and how much of the ingredients you use, etc... let us know nernan! 
I added about a teaspoon of topping to a small amount of bc, and it tasted yummo, but it did change the consistancy as we all thought it would.
Maybe adding 1 tbls milk 1 tbls topping and a little salt. This combo could be endless, strawberry, vanilla etc.
One other thought i had was using Ice Magic(what its called here) the chocolate that you squeeze on icecream then it goes hard, using it for writing and decorating. I am a choc and icecream freak if youhadnt guessed, blessed with fast metablisim.
If anyone else tries it and finds another way of mixing let us know.
Here's a Chocolate Syrup Frosting that is in the Recipe section. Sounds YUMMY!!
http://www.cakecentral.com/cake_recipe-2033-3-Chocolate-Syrup-Frosting.html
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