I often get wedding cake clients who come to me and tell me that another baker has told them that something can't be done. Well, maybe it can't, but it usually can.
There are definitely things that can't be done without fondant, but for the most part most things can be adapted or altered to get the effect that you want. It seems to me that a lot of bakers say "It can't be done" when they either mean "I can't do it" or "I don't want to do it". I'm not saying that everyone does this, but I've met my fair share of weird explanations of why things are "impossible" when it's obvious that they're totally possible with a little effort and/or creativity.
I recently had a client who wanted a real tiramisu, not a cake, in the shape of a Hokie for a groom's cake. The Mother of the Groom, who was trying to order this fairly normal (to me) dessert, couldn't find anyone who would tell her that it could be done, but they were obviously just thinking of a tiramisu and weren't even trying to accomodate her.
I suggested doing a tiramisu that had little figures of Hokies on it, rather than one big figure. I did an authentic tiramisu with homemade ladyfingers on the inside, and coffee pound cake on the outside instead of ladyfingers. That was to match the top layer of the tiramisu, which was a layer of coffee pound cake cut to fit on the top of it. The little figures went on top of that, and the problem of Hokie figures sitting in a mascarpone custard was solved.
So when your baker tells you that something can't be done, ask why. It might be that the design you want just won't work for some reason (fondant not buttercream, etc.). It might be that their skill level isn't up to it (in which case they're doing you a favor by saying that they won't do it), or it could be that they just don't feel like figuring out a different way to do things. If that's the case, go find yourself a different baker.