That wouldn't work. The reason for the rule against using the resort lots for theme park parking is partly about money, yes. They want offsite guests paying those parking fees in the theme park lots.
But the much more important reason is the size of the parking lots. GF has about 900 rooms. Polynesian, roughly the same. CR and BLT together, 1100-ish. So all together, the monorail resorts have close to 3000 rooms. With parking lots that are sized appropriately for the number of guests at each resort, with a little wiggle room for people coming to visit and dine and go to conventions.
That means that there are guests in 30,000+ Disney resort rooms that are NOT on the monorail. Most of them would love to park in a monorail resort lot when visiting the MK or Epcot. If even a few hundred of those cars descended on the GF parking lot one day...yikes. People who are paying hundreds of dollars a night to stay at GF can't find a parking space. People who are paying big bucks to attend meetings at the GF convention center can't find a space.
Each resort wants to ensure a good experience for their own paying guests. The GF management doesn't care if a guest in their parking lot paid to park at POR. They don't get any of that money, and all that car is doing is taking up a space that one of their guests could be using. They're happy to have you there while you're spending money in GF shops and restaurants, but once you've moved on to a theme park, they want your car gone, too.