I never found a manageable solution to determining how much of an area a PC had discovered.
A couple of things that were done in Vives 1:
- Count the number of times a PC has entered an area. And once a threshold is met, reveal the map for them.
- Provide challenge ratings for areas, and once a PC exceeded the challenge rating by more than 5, reveal the area for them.
- Always allow a flag on the area which disallows an automatic reveal, and always require 3 visits minimum to an area.
This more or less works on a PW, because once a PC has been through an area a number of times, the PC feels like they should know the map anyway.
It is however not what you are looking for.
Your suggested method of placing invisible objects throughout the area is a possibility. You could generate an invisible object in the center of a each tile, on the first area enter, and place an AOE on each of the objects. Sometimes things land on tree foliage when this happens, but it would more or less work and not require a ton of data entry.
Alternatively you could place map nodes throughout the area identifying all of the important things. On first area enter you sweep through and apply AOEs to each so that you can track when each is discovered for the player, and when all are discovered you flag the area as forever after revealed for that PC.
Anyway, those are a couple ideas.