Well, I just finished pulling the rear cover, and same thing: one screw out, one loose. I did not put longer screws in, as the threads seemed to have plenty of engagement. I did, however, clean and apply Loctite to them. My guess is that since the screws that are loosening up are directly above the exhaust port, the heating and cooling of the head and covers in that area might have something to do with it.
This whole thing makes me wonder if I should pull the covers on 901, even thought there is no seeping as of yet.
BTW, Bob, I got the logos, they look great! Can't wait to apply them to my van!