My PMI I-DF came with a plastic sight rail that slips over the barrel/bolt tube behind the direct feed port. I see no reason it wouldn't work on a PG or PGP, but it's probably usuitable for mounting any other accessory. I believe it was meant to work as a rear sight for the PMI's front bead.
If I can remember, I'll post up some pictures this evening. It says "P.M.I." in big letters on the side. They can't be that uncommon, but I've never seen another like it, though I've tried searching the Internet.