AFAICT it's about as dynamic as you get with a microcontroller -- at least, one that doesn't have external flash storage. And even then you'd need a way to get the new keymappings into the flash. The SpiffChorder design uses the 8k of flash onboard the ATMega8, which can be altered using ISP (in-system programming); we could probably figure out a way to make the USB connector pull double-duty for ISP and power/data lines, but it'd require a fairly major redesign of the circuit and I'm not sure it would fit into a pair of 28-pin wirewrap sockets anymore (which is a Major Win feature for me).

That said, we could probably use gEDA or EagleCad and the existing schematic to design a breadboard with surface-mount components, which would be a little spendier but would look really awesome. Can I have that rework station I keep asking for? :D