I know for a fact that Dallas had refused other teams the right to Interview Garret when he was OC, because they eventually wanted him as the HC.
Of course, the pressure of having to continually deny lead to Garret moving to HC quicker than most in Dallas wanted... so it did have an impact.
Refusals do happen, but generally when a Franchise feels a guy is 1-2 Years away from really being a solid HC and they want him to be THEIR HC, so they Refuse with the agreement that the Current HC will move on in 1-2 Years and the job is the Coordinators to have.
I think that Coaches should have a "Happiness" Gauge. Location, Win/Loss, Play Offs and Pay seem like 4 good items, and maybe Synergy (an OC wants the same Offense as the HC, for example, or close to it). This will determine if they take an Early Offer from the Team they are on. It's Yes or No, you give the Offer and they take it or test FA.
This way we have the CHANCE to lock these guys up, but it isn't cookie cutter in that every team is locking up every coach.
Maybe 'Time Together' would count as well, you rarely see entire Staffs break up, unless the HC is fired. Sure, you might lose 1-2 Position Coaches, who are moving up (ST to WR, WR to OC, OC to HC, etc)... but no one loses an entire Staff in an Off Season without the Head Coach being let go.
This game is great, but I strongly feel some RPG elements into Coaching and Player Development would make it even greater. Things like First Choice Offers for Coaches could help with that.