I'm not that great a programmer, but this one seems easy to me, you write an interpreter in brainfuck that allows function definitions that support recursion, then include your recursive function definitions as an included constant then point your interpreter at the constant and tell it to run it. I assume bf allows constants but like I said I'm not that great a programmer.

Obviously using a recursive descent parser is out but thats probably easy to work around right?

I could be wrong tho so don't base mission critical code on this without rigorous testing!