Thanks for the replies so far, guys. Reading the responses, I realized I might not've been so clear as to what I'm looking for;
A way to run a USB-ONLY keyboard into a keyboard with ONLY 5-pin midi inputs and outputs, without the use of a computer. In the same way you'd connect a MIDI keyboard-out into the MIDI input of a keyboardless synth, but with a USB-B "printer cable" instead of a 5-pin MIDI connecter.
The more I think about it... the more it sounds like a pipe-dream!
Then again perhaps I didn't quite understand some of the solutions - correct me if I'm wrong :)