Since the Create sucks big power as it is, the addition of an mbed shouldn't be a problem.
While I am quite deep into Forth these days, development may be better suited for eLua (since it is free). Maybe? Something to think about.
Of course, the SiLabs C8051F340x is also fully 5V tolerant (including power supply) with fewer external circuit requirements and much lower power consumption. I can't run eLua on it, but I've got my trusty SwiftX 8051 Forth....
So many options, so little time.