Thanks everyone. Seems like in this particular case I'll be stuck simply using a faster film. I wanted to go wider on the lens but a good vantage point for the overall composition was impossible (and the moon is only part of the picture I was going to make anyway). I guess we'll just have to see how this comes out. I've never liked 400 speed films in 35mm, but TMY2 is extremely fine grained so maybe it will work.
2F/2F, I have an aversion (rightly or wrongly) to that kind of "fakery" - although in the end one could probably argue everything in photography is fake. But for me making an image out of two separate images is the proverbial line in the sand. I have no problem with that kind of thing in abstract work, but I can't accept it in the straight representational photography I do. The same goes for exposing a roll of moons and then reloading the film to reshoot landscapes. I can only work with what's in front of me when I make the exposure. One of my many faults I suppose!