I don't think that's possible. A private/unpublished bridge, yes. But if you just run a relay with ... PublishServerDescriptor 0 ... I don't think that it

I don't think that's possible. A private/unpublished bridge, yes. But if you just run a relay with ... PublishServerDescriptor 0 ... I don't think that it would get a guard flag, and so your tor client wouldn't use it. But then, I haven't tested that.