"There are some original pre-ban 7.62x39 magazines out there, which work properly."
I don't know about this personally, I am just repeating something I once read on AR15.com. But, this guy claimed that he had a couple Colt 7.62x39 magazines and they were exactly the same as a 5.56 AR15 magazine down to the last dimension with the only difference being the caliber stamped on the floorplate. Again, this might be more internet hogwash, but that is what he said.
"Oly Arms also offers ARs in 9mm, 10mm, and .45 ACP that I know of for sure. Not sure if .40SW is on the list."
9mm AR15s have been around for years. Colt made (or makes ?) a submachine gun in 9mm based on the AR15. I own a pre-ban Colt Lightweight Sporter in 9mm. The 9mm gun differs from a normal AR15 in several respects. As was mentioned, there is a magazine block: the magazine is much shorter in 9mm so a block in installed to take up the space. There also is no gas system: the gun operates on blowback. And, the bolt is different. But, you can turn your standard AR15 into a 9mm pretty easy.
The Colt 9mm use a Colt magazine which are now pretty expensive. A 30 round mag costs over $100 if you can find one. Many of the other companies like Oly use an already existing magazine like the Sten or Uzi magzine. I don't know what they are doing in .45 ACP: maybe an M3 magazine ?