Will they be accessible under xexloader/similar- most likely (the 360 is pretty good with USB devices, better than the wii and certainly better than the original xbox. Not sure if I would call it PC grade but it is not far off).
Will you be able to use it to run things- certainly.
Will you be able to use it in lieu of a "standard" hard drive- not as such.
Note that a JTAG hacked box (more specifically one with the rebooter apps) can use any sata drive though.
A fairly short thread that might be of interest:
http://gbatemp.net/index.php?showtopic=210321