Happened to me too!
It was great fun trying to get the dropped screw back in under the hot exhaust pipe while wearing gloves (too long in a 'health and safety' environment!)
I like the idea of swapping for a proper bleed nipple though.

All mine years with XT i never used that bleeding, i open the filler cap and check that oil is coming from engine and then everything is ok.

There's really nothing to "bleed" It's just to verify oilflow, as in better safe than sorry. I've never had problems, wheater it was just an oilchange, or a rebuild.

Just loosen the screw lightly, when oil weeps out, tighten it up, and youre done :)

surely you need to bleed there so that there is no air trapped in the oil filter??

Why would it be "trapped"? I think it just gets pumped through the system with the oil, and ends up in top of the frame where air is already present. Otherwise we would see a lot of siezed engines.