Not a bad idea.
I don't like the idea of keeping a dedicated pouch on the weapon full time. For me, that just gets in the way. I do keep a pouch on the stock of one of my carbines to keep the mag that would go in the weapon if needed. If that were to happen, the pouch would be removed (quickly) AFTER the magazine was inserted and checked for seating, the charging handle was pulled, the weapon put on safe, the Aimpoint covers flipped up, the Aimpoint turned on, the SF tip off cover flipped down, and the SF activated once (with my head turned away) for a function check. Damn, I should switch to a simpler setup for "grab and go" (as in go downstairs and investigate).
Your idea is even more elegant because you don't need to yank the pouch off.
IIRC, Clint Smith at Thunder Ranch had an article in a gun-rag that had pictures showing a 30 round magazine rubber banded (Ranger bands or cut up inner tube) underneath the handguards on a carbine. This was to carry a spare magazine.
Your idea seems just as reasonable.
JAW