Not really related to openwrt, but I did install a chrooted debian distrib on my usb2 hard drive, and compiled mldonkey p2p program inside my router.
One you have apt running, compiling is quite easy. (dont forget to have a swap)
Everything went fine.
mldonkey takes a huge amount of resource about 40mbyte of ram (after some hours) and lot of cpu.
The throughput is not too bad, although it is certainly lower than on a more powerful pc.