Adding whipping cream to buttercream is pretty much what I did. Only I added sugar free white chocolate pudding to my cream before I whipped it.
Anyway, be careful about colorful buttercream on the whipped topping....it might bleed into the whipped part. Mine did when it came to room temp.
If at all possible, try to do the decorating with the same whipped stuff you use on the cake. Maybe save some out before it gets too lite when you're adding the cream?
Just a suggestion....please share with us your results! I'm hoping someone figures out a good recipe for this! I've not found one yet that I'm thrilled with and don't want to have to buy a ready-made product.