Quick summary:

(1) The default/Hijack kernels do not include RAID support, but it can be configured with a custom kernel build.

(2) The empeg filesystem / startup would require some minor hacks to make it work.

(3) It won't help with anything, other than avoiding loss of tunes when one of the two drives dies.

(4) Uploading tunes to the empeg will be slower; system startup will be slower.

Cheers