Marymoor does indeed have loaner bikes, and a very good beginner program. Check their facebook page and contact Amara Edwards for more info.

As to frame design, yes, there are considerations for track frames that are different from standard road, but that doesn't stop many people from converting road frames to race on the track. For local builders, yes, there are many (myself included) that could do a custom job for you. There are also several production frames that would suit as well. I might steer you towards one of the production models from State or All-City as a first go, before going the custom route.