I'm not positive, but I thought the gun/frame had to be originally manufactured as a "pistol", as opposed to a rifle with the stock removed. I remember seeing AR's marked up here as "Pistol Only".
Not that US law has bearing on our laws, but if AA were to manufacture a <16" .50Beo pistol in the US I think it would go a lot further to being marketed and sold as a "pistol" up here, making it a handgun commonly available in Canada, and thus (specifically marked) 10 round pistol mags available. They already make a 12" upper for SBR builds. If they mated it with a "pistol" lower and perhaps marked the upper as "pistol", well there's your 12" .50Beo pistol.
Given that the charm of shooting it would likely wear off quickly, a Canadian owner could later add a stock to make their "pistol" more akin to a .50Beo "shorty" up here. (AFIK there's no law up here against putting a stock on a pistol, they way they have in the US)
I don't know how many "pistols" would have to be sold up here for it to be considered commonly available in Canada. Yes, it would mean some number of people would have to buy expensive "pistols" that possibly wouldn't see much use in order for the rest of us to be able to buy the pistol mags.