his missing two years of memory (which haven't been mentioned for years - I hope it's not been forgotten about)

There was at least a teensy reference to it here: Adam commented on some of Jack's memories being absent.

I believe the boring real-world reason that it's never been followed up is that RTD considers that particular plot point to be the property of Steven Moffat, so unless SM ever writes a TW ep or another DW with Jack in it, it's stuck in a metafictional lay-by forever...