The first time I saw an AR that could have had a folding stock, although it didn't, was an AR pistol I saw at a gun show in San Diego circa 1998. Olympic Arms OA-93. It has the operating rod and return spring in a tube above the barrel.
http://www.imfdb.org/wiki/Olympic_Arms_OA-93As soon as I saw it, I thought, "Woo hoo, now I could make an AR with a folding stock!" And then kind of got over folding stocks, because fixed stocks are generally lighter, stronger, and more comfortable. But anyway, if you wanted to, you could take an OA-93 and add the folding stock from the Oly carbine, and there you go. (Don't blame me if it sucks, though.)
http://www.quarterbore.com/ar15m16/oa93.htm