It might have been rumored that Maser is the OC.
It comes down to this - the NFL doesn't work the way it used to. You have to have permission to make this sort of move, and Carolina doesn't have to let Maser go. Some teams' head coaches/GMs don't allow guys to go for coordinator - type promotions, though guys like Brian Billick have (JDR, for instance).
If they do let him go, Magazu was a long-standing line coach, and they could pick quickly from amongst the TEs coaches available. It wouldn't be a devastating loss. Personally, I don't see where Maser is that important a choice - it's really just a way to go get an OL coach who's under contract, since Coughlin will call plays himself. That makes Maser a guy who runs the offensive practice and does some run gameplanning.
And yes, having agreed to be the OC for the Giants, or having talked with Coughlin, would likely be considered tampering.
The rest don't stand much of a chance of leaving. Were there many college HC openings available, I'd say Skipper would be a consideration.
O'Brien's physically unable to go anywhere. As assistant head coach, there's absolutely nothing any team could offer him other than a head coaching assignment. Though that largely applies to anyone, including Maser, O'Brien's the only coach who likely has absolutely no chance of ever leaving.
Williamson might be a consideration, only because he's been at this a decent amount of time and retirement becomes something to think about.