you can do this manually by running mksysb to filesystem/nfs (see smitty mksysb)
mount the nfs filesystem on the linux-box and copy it to the tape, don't ask me how, manually copying to tape was before I become an admin
we use tsm for this
this just works for rootvg
but if I get you, you want lanless backups, directly to tapes?
nope that is not possible, since
1. tape virtualization is not possible jet, so you need dedicated hbas an each lpar for this, I assume you use an virtual io-server and map disks/lv
and
2. you can not share the tape drive from your red hat box over san to the lpars, but I'm sure addidional software is able to do this
but then you are facing point 1
If you want to directly backup from the ds4700, there is a mechanism called flash copy of the luns, but you can't copy this to a tape drive
we tried this with a ds4800, and almost had a whole outage of all systems running over vio, so better don't try on production environment