Jeff, I have one comment that has value only very early in the design of your architecture. Since you've made a choice of C as a implementation language I suggest that you add one more target to the list of supported platforms: standalone a.k.a. bare metal.

It will be nearly impossible to convert your code later unless you very early make a commitment to supporting the event-driven programming like using lwIP-style network interface in addition to socket-style networking.

The same architectural choice needs to be made early to accomodate various lightweight flash-storage access methods instead of Unix-style open/lseek/read/write/close. Same for fork/wait/pthreads versus any RTOS-style explicit synchronization primitives.

One possible advantage of such choices would be that instead of relying purely on donations you may be able to obtain funds via cooperation with future manufacturers of the future Bitcoin devices that will greatly benefit from the "very low resource usage".