Updated to cpchan's latest build and it works perfectly.
Surprisingly there's less of a slowdown now when navigating through menus while playing music, even though I didn't see a patch note anywhere which says that this was fixed.
Well, I'm satisfied now. The way Rockbox now runs on the Sansa is not yet perfect, but it looks like I won't have to use the original firmware anymore.
By the way, including the "playback" menu in the wps's context menu is a very good idea. This should be considered to be included in the main branch.