So when I lived in VA, there were a few places that held meets every weekend when it was warm outside. So any where in portland or anything or Salem where people meet and hang out. The Bass pro hosted the meets in VA, so it wasn't against the law or anything.
I don't know if you're willing to make the drive to the Portland area every weekend, but every Friday and Saturday night a lot of us on Stangnet meet at the Fred Meyer shopping center off highway 217 and Beaverton-Hillsdale Hwy.(Highway 10). Sometimes at the Sonic burger off Cornelius Pass south of highway 26 headed to the coast. And every Friday as well, the Dairy Queen in Aloha (Kinnaman and Farmington Rd. - further west down highway 10) has a cruise-in in the early evening.
If you're looking for other events down your way, check out driveonline.com and mustang-club.com. There might be some stuff posted in the event sticky here in the NW forums on stangnet.
There is a new Mustang club in Eugene that just formed last year. Their website is www.eugene-mustang.com. I think they hang out at a Carls Jr. every Friday night. That might be closer for you than Portland, Beaverton area.