Well, after some waiting, I've now decided that Xee is good enough for a real 1.0 release.
http://wakaba.c3.cx/releases/mac/Xee1.0.zip
1.0 features some more bugfixes, a full set of icons by James "Kibo" Parry, and a Japanese translation by Hinata. Many thanks to them, and also to MrVacBob for being an invaluable help for me while trying to get a grip on Objective-C and Mac OS X programming.
There is now a separate thread for general support questions for Xee, which is also linked from inside the program itself:
http://wakaba.c3.cx/sup/kareha.pl/1132091963/
Source code is available, for those who want to hack or help out: