What you have heard about chairs at Fort Williams is correct. They allow like 10 chairs or so on the grounds for ceremonies. I rented the picnic shelter for a number of reasons, but chairs was definitely one of them. That guideline is pretty much true for every park in Portland too. They don't want a bunch of chairs putting holes in the ground.
Another option would be using the amphitheater, which has stone seating - you could put blankets down so people weren't sitting right on them. That spot is tough though - depending on the time of day, the sun is brutal. We tried doing pictures there pre-ceremony around 2 (mid-July) and it was squint central.
If you go here:
http://www.capeelizabeth.com/visitors/attractions/fort_williams_park/home.html and click on the "Wedding/Partnership Ceremony Request" you will see the statement about chairs. It doesn't specify a number, but it does say for elderly and handicapped only.
Picnic Shelter Ceremony:
The ceremony was on the stone "patio" area off the front of the shelter.
Amphitheater: