I'm not sure there are any sources for the kind of data you're talking about. I really wish there was.
There are a few sites that are user based that were trying to build up a database of gpx files of waypoints/routes.
This one - trail.brijn.nu/index.php
- Didn't get very far along but the mapping part of it was pretty cool.
There is the ability here - www.clubtread.com/Routes/
- To upload gpx files, and download waypoints. No downloading of route files yet, but it's supposed to be coming soon. Club Tread is pretty active so I expect the database to get pretty large after awhile.
Need to be a paid member to access most of the info here - www.bivouac.com/
- Lots of waypoints available. Mostly used by climbers, but lots of details on road access to pretty much anything climable.
Haven't used this site much - www.trailpeak.com/index.jsp?cat=def
- If you join and share gpx data with them, you gain the ability to download from them.
None of this is quite what you're looking for and none of them are compatible with my gps like your project is, but there is some info availble to you out there. I'd love to see your project grow and I've been checking your website on and off since I first saw your post on the gc.com forum.
Hope some/any of this helps.