Maybe if for any hiatus over a specified amount of time, you need to discuss it with the mods specifically? Like 'you take take this much hiatus time without a reason, but for anything over that you need to tell us why', or something.

Though in regards to some people just punching the clock - that happens in any game, regardless of how strict the AC is. I've been in games with much more stringent ACs that still get people character squatting. (Perhaps players should be able to go to the mods if they have a concern about a particular character being squatted, to request that they look at the situation (and if need be, talk to the squatting player)?