Having done the trip now I'll answer my own questions for the benefit of anyone else thinking of doing similar...

I arrived at 21:45, checked in to the Transit Hotel (which was perfect for what I needed - very comfortable bed (you need to like them firm!) and a decent bathroom with a bath.

Then I went out for a nightcap or two, and I can confirm that the JAL F Lounge was happy to grant me access at 22:00 or so on the basis of my CX F boarding pass for 10:35 the next morning, with an apology at the desk for the fact that they would close at 1:30am. I didn't stay that late, but I did take a few drinks and tried the teppanyaki beef plate which they had a chef making to order until 11pm (perhaps 11:30pm). There were two champagnes available - one (Laurent-Perrier IIRC) in the strange room at the end which had a shoeshine at work and a fussbal table!- and the other (a less well-known name, sorry I forget which) in the main food / bar areas. The ambience is OK, if a little brightly lit for my taste at 11pm; it was probably 10-15% occupied at 10:30pm, diminishing from there.

In the morning I went to the CX lounge, where they had a choice of Japanese or Chinese set breakfasts, plus a small selection of cold cuts, fruit, etc. I went with the Japanese set, which was solid (for what is essentially a J lounge). Juice and coffee obviously are readily available, as was a full bar, but I decided that I wouldn't get back on the booze until I was on board! I like the ambience of the CX Lounge - it's the same as all the most recent renovations (e.g. Pier F), and it has a great view over the airfield. That being so I didn't bother to go and check out what the JAL F lounge was like in the morning, but I guess it would be more crowded.