I really feel like this might have all started from some posturing by Danny to get Posey signed. Think about it...
Danny offers Posey less than the midlevel. He doesn't want Posey to sit around and wait to see if offers get better after the big FAs sign. So he "offers" Maggette the midlevel, thinking it's unlikely he'd take it, but it's more realistic than a guy like Baron Davis taking it, so more believable to Posey and his agent, hopefully scaring them into taking Danny's offer for fear that it won't be there in a few weeks. The Spurs hear about Danny offering the midlevel and giving the "championship" incentive, and figure they are in the same boat, and offer the midlevel to Maggette. Maggette, gets the offers, says he's "interested" because he wants to scare teams out there with money into offering it to him now, for fear that he might not be there when all the other big names sign.
Now of course Maggette could be interested in joining the C's. But it seems unlikely. He's 29 in November, if he signs any contract of length, this could be the best it gets for him for the rest of his career. I know we talk about sacrifice with the Big Three, but remember, all they sacrificed was stats, none of them sacrificed any money.