If there is going to be an universal resort reservation system, then I'd expect no company to be able to build/adjust/test a fully functional system by this current ski year. I'd expect many bugs to be worked out as the 2020 ski panned out if there's going to be a ski year
I know the answer is "IT system" but I guess my question is why?
You need an input number of slots available each day - at the beginning this could be manual e.g. someone going through the calendar and determining a number of slots
You need to validate pass no against the pass database
User then picks slots available
Pass is whitelisted for the dates booked (strictly needn't happen until night before)
Difficulty comes when you try to get dynamic around slot availability and start doing all sorts of modelling in system but on a basic level something could be built.
Wouldn't be entirely fair as one would expect manual slot number to need to be somewhat prudent and hence availability might skew to "week of" reservations. Which for "planners" may feel somewhat unfair.