Just to let anyone interested know: I came across sawfish-mmc [*], and while it's not very usable right now (drawing bugs, bad configuration menu, and maybe more), it does solve my focus problems, and it also has better keyboard handling for the menus. I hope it'll be possible for me to split out those modifications from the rest, though it'll be hard since there have been a lot of other changes (and since I'm not very familiar with sawfish internals yet).
[*] http://maruska.dyndns.org/comp/packages/
