Linux usually uses scsi emulation for ide drives anyway, so maybe Puppy should normally just boot from a scsi drive with no problems. I don't really know, i have an ide drive. Actually, I think Linux usually has better driver support for scsi than ide.
the PMEDIA= boot option is just used in the Puppy init script, written by Barry, and that variable was not used in the Puppy 1.x series
you could post your questions on the Puppy forum, where Barry is likely to see it