Well I finally had time to try the LaCie Rugged Safe upgrade, and I can say for sure that it *is* possible to change/upgrade the drive in this enclosure.
It was actually surprisingly straightforward:
I removed the rubber bumper, opened the case (latches--should be pretty obvious how to pry them open once you see them), then unplugged and replaced the drive inside.
It's a totally standard 2.5" drive; it appears that all the authentication/fingerprint recognition stuff (and even the fingerprint storage) is handled in firmware--none of that seems to reside on the actual drive.
The case appears to even accommodate pretty thick drives (the 1TB Toshiba in mine was almost 12mm high).
I placed an HFS+ formatted disk into the enclosure. Before swiping my finger, it mounted no volumes (so it appears that any existing data on the drive is wiped out; also the small partition with the software and documentation is gone--I'm not sure how to bring that back, but I don't need it anyhow so it's fine for me). After the finger swipe, the drive presented itself as unformatted, and I could use the MacOS Disk Utility to format it.
(I did not try making multiple partitions, but this should be possible too.)
I did this experiment with a 320 GB drive, so I cannot say what the upper limit of drive size is that the controller can handle (am planning to try a 2TB drive soon).