Interesting. A while ago I was thinking about a similar solution - machining a dust cover that would bolt up to the receiver with an integrated pic rail.
But of course like most of my "solutions" it never got past the "thinking of this" stage.
The dust cover itself isn't really the problem, though. So if you've just figured out a way to lock it down effectively then that is probably sufficient.
My only real complaint with these rifles is that you can have a forward-mounted optic, but not really much else unless you are satisfied with the POSP-type side rails.
I would be really interested to see an integrated rail that goes from a locked-down dust cover to the rear sight (assuming this could be done without turning the rifle in to a stovepipe machine).
It would really be nice to get enough space to put a red dot and magnifier on. Even though the x39 is a 200m cartridge IMO, still, a 3x flip-mounted mag would be nice for the 200m shots. You MIGHT push it to 300m, I guess. Well, some people might, anyway...I am skeptical of my own ability to do it.