I'm trying to think of ways that the everyday guest could FUBAR this, and I'm struggling
There aren't necessarily good ways of 'resetting' demand. Is it just when the next bus gets there? how do you know the bus was able to actually take everyone who was waiting?
Plus, not all guests have room keys.. (so now you have ticket media to use..) and do you expect every person in the party to swipe their card? That's a disaster.. and you can't just let guests enter how many in their party.. they'll abuse it.
'Per Person' queue monitoring is problematic and overly complex. Disney just needs to monitor frequency of stops, and monitor when there is abnormal demand that is overloading their normal capacity planned.
They already have dynamic capacity handling and can add remove/buses as needed.
As long as they have regular stops - the best data collector is already available.. the human bus operators. Bus operators can simply provide feedback to the system that there may be abnormal queues, or pass customer complaints simply with location/destination info which dispatch can match against known activity.
It trumps my idea of having CCTV cameras at every stop, which would allow dispatchers to see crowds at each stop. However, that would only work at the parks, because each bus stop is for a specific destination. They could implement YOUR system at the resorts themselves...and for guests who are just touring resorts or dining, they could also swipe their park tickets to validate themselves.
Anyone who doesn't have a park ticket or room key can just sit and wait for a bus that's going to their destination. That population would be pretty small though.
Regardless, they seriously need a way of knowing how many people are queued up for buses at individual bus stations. Because right now, the "guy in the white van" is NOT cutting it.
Why do they need to know exactly how many people are there? Attempts to try to make the system a true 'one to one' demand->capacity system will simply not be effective.
The demand system should be there to help provide feedback to the system when the normal routes are not functioning well.
You could take a simplier approach then the 'per person' model and simply have someone activate a indictator saying 'we want to goto AK' and that stays active until an AK bus arrives. But that would likely be a better reporting tool then it would be to better service day to day.. because unless you are trying to make buses 'one to one' with demand, you're going to say 'well there is another AK loop bus due to 10mins' and leave it at that. The feedback would be useful to alert things like abnormally long waits, 'back to back' waits (which would show higher demand then capacity), etc.
Simple is almost always better in these cases when it comes to 'user input'. Keep the guest input brain dead simple... keep the bus operator input simple so they can do it quickly and efficently.
