It would be cool if the state made a set of walk in campsites at Cathedral Ledge, but as was said before, I'm sure it would piss off the locals trying to get a decent nights sleep (remember climbers wouldn't be the only ones using it).
All in all the camping on the Kank (Blackberry campground I think is the closest one) is about 10 miles away from the cliffs, and that's really not too far when you think about it. Nice campsites, beautiful scenery and no rangers knocking on your car window at 6AM. And best of all it doesn't screw up access.
As far as Rumney goes, I don't really think the people living across from the parking lot want 100's of climbers calling them looking for campsites, so I wouldn't ask for their number to be posted. But if you just go and knock on their door at a reasonable hour (pre 8PM) and talk to them they will point out where you can stay in the field on their property. I think it's like $5 a night that they ask for, which is a pretty good deal. Try to get them early in the day and if they're not around just come back later. If they're still not around find another place to camp instead of squatting without permission on their lawn. There's a big campground in Franconia Notch about 20-30 minutes away or you could find some cute co-ed to share a room with in Plymouth.