It's not too late, in fact I'm seeing some fresh adult peacocks flying now. Although I've not bred peacocks I believe that, like most butterfly chrysalides, they will darken a day or so before emergence. You will need to make sure that it has something to climb up and hang on to while its wings expand (they need to hang their wings downward) so a jam jar is possibly not the best container to use. However I've never heard of peacocks on hops, they generally feed on nettles, so you may not get what you expect...
Thanks for the great answers! Although I can't mark this question answered yet, you've given me hope.
Clive: I trapped it when it was still a caterpillar, and it actually attached itself to the underside of the jar lid, so it's hanging down nicely. On mossywombat's first link, I see that hop is in fact listed as a foodplant.
mossywombat: thanks for the links! That's them all right.
I'll update this thread when I have a butterfly, or when I finally give up.
I said I'd update this thread when I finally gave up, and I'm afraid I have. The brown pupa eventually developed dark blotches, so we had to accept it had died. Here are some photos: http://www.flickr.com/photos/fjordaan/4961634980/ (I don't think I took any photos after it had turned brown, unfortunately. It was only this bright green for a few days.)
Better luck next year, I hope!