It seems all the other WRT54__ models get the love, why not the WRT54GC?
Some sources say that there are other firmwares.
A big problem is flash memory size and RAM size. It has only 1 MiB of flash and that's too small for most GNU/Linux distributions. Same thing for RAM. Take a look at requirements for DD-WRT. It just can't fit nicely in 1 MiB.