Well, given the penchant for classifying one firearm as being a "variant" of another, it is entirely possible that the AIA could be deemed to be a variant of the Lee Enfield.....
And if the AIA is deemed to be a Lee Enfield variant then the Lee Enfield magazine regulation would kick in. And you should read this very, very carefully.
It the AIA is not deemed to be a variant of the Lee Enfield, then the just another manually operated firearm's magazine situation should apply.
There is certainly a market for extra AIA magazines, for use in the AIA rifles. A 20 rounder is a bit silly in a bolt action rifle - was tried 95 years ago and was rejected. The obvious market for these is for use in the M-14 rifle.
As noted above, the AIA made 10 rounders do work in some M-14s. Have any of these new, non-AIA made 20 roundmagazines been tested in a number of M-14s? Have any been tested in AIAs? Bit of a pig in a poke, so far? In any auto firearm, the magazine is a critical component. Note that the distributor will warrant that these magazines will work in the AIA, but will not guarantee their functionning in a M-14.