>... would it just be a case of for every bit changed, we need to change another bit in the opposite way...
No, hash functions don't work that way (otherwise it would be too easy to crack)
The SHA-1 hash has not been cracked, it has been bipassed using a much simpler exploit (ie. a 'bug' in the loader)
> I read this can be done on the zune/something else...
Not true. The Zune has very similar SHA-1 logic, but they fixed the bugs so you can't use the same exploit. The GigaBeat "S" is wide open for homebrew, the Zune is not.
Other devices, including the iPod and some older WinCE devices, use simple checksums (for a simple checksum, it is very easy to change bits the 'opposite' way to fix up the checksum). They have been wide-open for homebrew for a long time.
> would there be any other issues to prevent code from running?
Not due to security checks. Figuring out how to directly access the hardware and port Rockbox (or Linux) is the work that must be done. So far the interest has been very low. That's probably because it is GigaBeat "S" only.