Is it possible to do brush embroidery (carefully) on top of crusted buttercream, or must it be a more solid surface such as fondant?

yes if your careful it can be done

I did mine on a non crusting white chocolate bc and it worked. I just had to remember to use light strokes and dampen my brush very regularly.

http://cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1560707

Thanks! I was hoping this was the case!