Having studied librtmp over the weekend, it doesn't drop into gnash as neatly as I'd hoped. Gnash already has classes to implement NetConnection and NetStream, which it makes sense to use - but librtmp is designed to replace the need for such classes to exist. So it implements certain higher-level responses to various calls from the server, that are necessary when downloading a stream. Gnash probably needs to hook in instead, and let the flash application decide what to do in these cases.
So more work required, and possibly changes to librtmp in future - but I've started fixing up Gnash's RTMP code instead, for now, and have sent a couple of initial patches to Savannah. I don't think YouTube exercises these code paths, so they're a little rusty - I hope to make more progress through the week.