However I was thinking that the "only_tor" mode for tahoe (mentioned
here https://tahoe-lafs.org/trac/tahoe-lafs/ticket/517#comment:15 )
could be implemented in tahoe... it could filter endpoint descriptors
by calling clientFromString or serverFromString and then checking the
object type.... or is this a terrible way to do it? I'm not sure. The
thing is... tahoe wouldn't normally
know anything about endpoints because it uses Foolscap which could use
endpoints (it does in my branch
https://github.com/david415/foolscap/tree/endpoint_descriptors_server2).

That sounds pretty ugly. If you want a specific string parser that only calls your own one specific endpoint type, perhaps don't bother querying for plugins at all, just call your own directly?