The good news is that, despite the buttercream frosting-only style of the bakery (which tastes better anyway, right?), they did have one cake which was about as close to what we were looking for as we could possibly have hoped to find without asking for a custom design... it looked almost exactly like this, but without the dots:
And it was made out of buttercream and chocolate frosting! No bleh-tasting fondant!
I'm going to meet with the baker again a week from Sunday, armed with a few pictures of henna/mendhi cakes and of hennaed hands, and just let her go nuts. We're getting two tiers of brandy-soaked yellow cake with pineapple custard (way better than it sounds) and a tier of white cake with vanilla custard and strawberry preserves, and we're getting it for under budget. Yeah, so I don't get my marble cake with fudge and bavarian cream... but whatever. At least I know this cake will taste amazing. ;)