If I recall Savage for a brief period made one in their Model 25 rifles but don't quote me on that as I'm not 100% positive. I know someone either does or did make an AR Upper for the 5.7x28 and I think some of those use the FN PS90 50rnd mags.
The 5.7x28 is a really cool looking little cartridge, I have a FN five seven pistol, but the brass isn't the greatest and kinda sucks.
Not that 22 Hornet brass is stellar but if you're going to reload for this I'd go 22 Hornet all day long over the 5.7x28 and since I'm no particular fan of the standard 22 Hornet if I was going down this road I would do either a 22 K or Ackley Hornet.
What about a 20 Caliber version K, Ackley, or Hornady Hornet. I've always thought one of those shooting something like the Hornady NTX 24gr or 32gr Vmax would be a fun project.