Well, even if other GMs have assumed this, and I'm inclined to believe you're right about this, I would also think that the other GMs would send some whispers into Moss' or his agent's ears in the name of gamesmanship in an effort to drive up his asking price given that this is indeed a league with a finite salary cap. IOW, if you were the GM for the Jets, the Bills, Colts or SD, wouldn't you try to ensure that someone of Moss ability sign with the Pats at market rates rather than sign at a hometown discount rate even if you were not seriously trying to sign Moss?
I also think that when Moss took his pay-cut to come to Foxborough, there had to have been some good faith, gentleman's agreement that he'd not be tagged at the conclusion of the season and that every effort would be made in good faith to resign him between the two parties involved. To me, this is the only logical explanation at this point and given what little we know as of now on why Moss is not seriously fielding offers from other teams. I just think the two sides are deadlocked, and I'll speculate that more than the total sum of the contract, it probably has more to do with either the length of the contract or the guaranteed money.
Even at this stage in his career, he still is one of the top, if not the top, FA. Consider also the WRs who've already signed, which further dilutes an already unremarkable FA-cy class, and considering also that the strengh of the upcoming draft is hardly within the WR corp, a team interested in adding a prime receiver has to be interested. When Philly is rumored to be interested in trading for Fitzgerald and his contract, then I would think they have to interested in Moss as an alternative.