In KDE 4.4.x, you could use the environment variable KDEWM to change the window manager (e.g. export KDEWM=openbox). But in 4.5 and 4.6, using KDEWM will cause the Plasma workspace not to work right - menus and popup dialogs will always appear offscreen. There is a workaround - to create a new startup item that kills KWin and replaces it with the other window manager - but that's rather ugly.

Are there other workarounds for this in 4.5 and 4.6? And what are the current plans for compatibility with WMs other than KWin?

These problems shouldn't occur as such, KWin shouldn't be needed to ensure the system operates properly. Try setting the Window Manager through System Settings instead.

Apologies for taking so long to reply on this...

The answer is, using the Default Applications page causes exactly the same problem. This happens with several different WMs too, so I don't think it's a window manager bug.

Edit: Hmmm, it works mostly fine in Slackware with Fluxbox though. I'm beginning to suspect that this has to do with some of the patches that Debian-based distros apply.