The only reason I can see for not having more than 10 is if you own a single stack 1911 and don't want it to get really awkward from the extention hanging out of the grip.
Nah, then you just need to get the .22 conversion for it. You could fit 20 or so rounds into a single-stack .22 magazine that still fits inside the grip. Or probably about 35 if you double-stack it. And no bellyaching about double-stacking making the grip uncomfortably wide, either; this is .22 we're talking about.
Actually, that raises an interesting question: It's illegal to 'make' a standard-capacity mag for a gun by modifying an existing mag for another gun such that it no longer works in the gun it was made for. But what counts as 'modification so that it no longer works in the intended gun?' If I have a .22 conversion for a (say) 9mm beretta, and I take the 9mm double-stack mag body and take the spring out, and put a (specially designed for the sake of this question, hypothetical) insert and new spring in it, it'd hold a heck of a lot of .22 rounds. You'd have to take the insert back out and put the original spring back in to use the mag for 9mm again.[0]
Hey, I guess if the feds complained about it, one could point out that they consider having all of the parts for a full-auto to be equivalent to actually having a full auto, so as long as I have all of the parts for a regular 9mm mag, that's the same as having an actual 9mm mag.
-BP, who shoots .40 anyway, and is mostly just woolgathering to avoid work.
[0] On that note, which would be better? 15+1 9mm rounds, or 30+1 .22 rounds? The 9mm would probably have a better chance of stopping the bad guy in fewer shots, but with 30+1 .22, and a spare mag in a belt clip, you could put so many holes in him he'd whistle when he walked...