I don't know if this would work or if it'd be acceptable in the germ killing area....but doesn't UV light kill bacteria? Or sanitize or something. I see those UV toothbrush germ killers in the stores.
If something like that meets the germ killing level needed, what about zapping the device between each patient? then there's no moisture, no alcohol and no damage?
Maybe it doesn't work fast enough, I honestly don't know.
Other than that, I can't imagine the alcohol hurting the aluminum back. But you can also get snap on cases for the back relatively cheap anymore. Depending on how often that iPad is handed off, could switch the back cover between persons, so they always get a fresh back (and you can clean/sanitize those between patients) then use the alcohol to wipe down the screen protector covered front.