What if you want to put the thermostat pre-set at 74 degrees in a clear wall mount tamper-proof enclosure and install a 6 hour timer next to it so the knuckleheads don’t leave the a/c on when they leave? Should I just break the G wire to the timer and back or what? Will that affect the delays the thermostat has for pumping out the ventilation system before it shuts off?
No one would ever do that.
They would get a programmable thermostat. It is trivial to set up programs that do exactly what you want to do. Further, many thermostats have mechanical gating so the up and down (temperature) buttons are readily accessible and changeable right now, but will reset at the next program time. Access to program is gated by a special door. You can then put a cage over top of it so they can access the buttons, but not the door. They typically allow 4 program changes per day.
This is a $50 solution, and you can't homebrew a thing like that for so little.
I've personally never seen a programmable thermostat that you can turn on whenever you want and have it automatically turn off after a certain amount of time. A programmable that I'm used to seeing would, for instance, always turn the A/C off at 6:00pm or 9:00pm, but not "6 hours from when it was last turned on". If anyone knows of a thermostat that has the specific function ask for, please post a comment.
Using a programmable thermostat would be the best option, but if you really want what you have described (like a hot tub timer at a public pool - turn on any time, always turn off 20 min later), then you can wire the Y and G wires together and pass those through a timer.
The Y wire calls for cooling and G calls for fan. If you only put the Y wire on a timer, the fan will still run all the time. If G is on a timer you could potentially run the compressor with no fan and it would freeze up and maybe cause some damage. The good news is that it's acceptable to just wire Y and G together. You loose the ability to control the fan separately and you will not get any fan "after run".
I've seen something like you're asking used in hotels before. They would have a regular thermostat that allowed the guest to adjust the temperature to their liking, but there was also a keycard reader tied in. In order for the A/C to operate, the guest would have to insert their room key into this device and leave it in. After removing the key (which tends to be a requirement when you leave a hotel room, as you can't get back in without it), the A/C would shut off.
An example I found by Googling would be here
This is a similar system, but it uses occupancy sensors instead of the keycard.