A 9mm BFG blank has a rim 10.6mm (0.417") in diameter, but a Glock mag, for eg, double stacks cartridges and, because the blank is shorter than a round including a bullet, it may well be possible to stack them with the rims staggered forward and back. You would have to modify the gap between the mag's feed lips to get the blanks in/out and you couldn't get as many rounds in as the mag is designed to hold, plus all the tension would be at the back between each forward stacked rim and the cartridge cases above and below which it pressed against, so they wouldn't be very stable inside. Although a mag spring is pretty strong so the main issue would be whether the instability down the stack of blanks would make the top one present at an angle which would make it slip out from between the lips without being pushed from the rear - I believe this would indeed happen and I doubt that the obvious first solution, to modify the mag follower, could be made to work. Nonetheless I suspect that it may be possible to design feed lips which could cope with wonky angled blanks, but the problem then, due to making the mechanism tighter, would be that those cartridges stacked with their rims at the back would not easily slide forward out between the feed lips, since the rim would be caught against the rim of the cartridge below and the only solution to that is looseness, play in the mechanism...
In short then I'd like to see video of someone using a 9mm pistol mag to carry and dispense BFG blanks, but until i see it, i say bollocks.