"one might consider a combo of a used scuba breathing regulator (Stage I&II) "
I would never trust used
equipment, of unknown origin and condition, to remain reliable for critical
emergency use. I've also seen "perfectly good" regulators freeflow after being in
storage for a few years, as the pressure setting invisible inside the first stage changed with age. Also wouldn't trust a used tank of any size, without getting at least a new VIP for it. And then, since most will also need a hydro...you're close enough to the cost of new to just buy new and *know* it is reliable. (Unless you're patient and knowledgeable and hit a bargain, including
shipping rates.)
4 liter, 6 liter...No really. Wouldn't you want something more like a 19-30cf tank, so you could duck back under and
salvage the abandon ship bag? Or some
food and fresh
water? Anything like that?
And don't forget, the
hatch and any escape aids should be marked or painted luminous.