it is not possible to put the window focus on editres with Windowmaker (neither
sloppy mode, nor click-to-focus). Additionally it is not possible to enter text
into any of the text fields (eg. Resource Box). I've tried it also with Openbox
as Windowmanager with the same effects.

Focus and text input works with twm, though .. so it doesn't seem to be related
to editres. Probably some setting which Windowmaker and Openbox honour, but not
twm.