I dunno of anything exactly like that (I have seen one somewhere, but don't remember where or what... ). But bittorrent clients are still evolving, and a firmware implementation would probably suck as far as newer features (DHT, peer exchange, encryption) go...
There's no utorrent under linux, but there's rtorrent. rtorrent's an ncurses-based client, that you can run in screen. Then you just drop your downloaded torrent into the directory and it does the rest. It also doesn't have a lot of the new neater features (like encryption and DHT), but it's still under active development, so it's good times. When you want to check on it, you just ssh in and hit "screen -r" and check. You can do ^-a d to detach again. It's good times,
check it out.
More often than not, you can built yourself a low-end, low-power computer with as much capacity as your NAS device would have for about the same price as the NAS, only you'd have a lot more flexibility with it. Also, hdparm/smartctl tuning can let you control drive spindown, letting you standby inactive disks automatically to save yourself power. My fileserver's cpu uses 23 watts, and all the disks spin down after half an hour idle time. It's just an athlon xp-m 1.66, but it's certainly fast enough for what it's doing.
Last I knew, an idle sempron 3000+ was about an 11 watt draw, peaking out at about 40W fully loaded... not exactly a huge hit on either your wallet outright or your power bill.