hi.
I managed to get a Koto sound out of an electric guitar with the Axe.
You'll need a compressor to get a real snappy attack, than use the Expander to 'kill' the decay. I guess a properly set up resonator could take care of the banjo specific uh... resonances. An abused reverb with a really short setting produces these odd metallic buzzing (combfiltering, I guess) - or use the comb filter instead - ha!
A custom IR to get from the DI guitar signal to banjo(esque) sounds might be another way.
If someone can supply a dry (only banjo, no reverb etc.) recording I can construct the IR.
It'll only be a step on the way, tho.