yes gentoox would work, but your bios/kernel probably wouldnt. (without half-ass working DVD drive)
I have heard that Xcromwell can be edited for use with two HDD's, instead of a DVD, and a HDD...but have never done this myself.
hard drive being unlocked is irrellavent to running linux, if your xbox will boot to an xbox dashboard, your xbox will run linux. (unless its a version 1.6 xbox)