I'm not sure if something like this has been written already...
The idea is sort of the reverse of The Chrismukk-huh, in a way. In this case, Sandy/Kirsten/Seth/etc know Ryan and he's been living with them in Newport for 1/2/3/4 years, but Ryan doesn't know them. I'm thinking of like a wrinkle in time type thing, where Ryan is the 15-year-old whose mother just threw him out (on the same day he meets Sandy for the first time, in the pilot). He calls Sandy for help, but the Sandy who answers the phone is from 1/2/3/4 years in the future and knows Ryan well.
Just a middle of the night plot bunny...
ETA: If anybody wants to adopt this, I've posted my beginning of it at
http://millstone1005.livejournal.com/36960.html