Looks like a good system so long as the airline allows a carry-on and a personal item of the generous size you mentioned. I have seen small (non-US) airlines that limit personal items to things like a woman's handbag or a briefcase. There is also often a weight limit.This may help.
You can carry technical diving gear for up to hypoxic trimix on board using this method. 5 sets of regs, wing, plate, harness, computers, 5mm wetsuit, 2 masks, trimix analyzer, fins, boots, gopro, stage rigging, 6 tank bands, and a number of other accessories.
The only thing I did not carry onboard was my AL40 O2 tank. That was in picee of checked luggage.
Just did it in fact to Bonaire.
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Also, maybe I missed it, but what was the rationale for not wearing the backpack while boarding the plane?