I would GUESS:

They have never even seen such and it will be up tp the FAs to make a decision. The United flight crew is much more likely to be a sticler for some regulations than the Air China flight crew.

On TPAC flights I always see passengers spread out any way they can. When I have to fly in PE, I will build a foot rest out of my bags and stuff.

All in all, I would give you very high odds of not having any problems,

Of course wait to well after departure to inflate it (I would wait till after the departure meal). And obviously do not book an exit row.