Esta pergunta foi respondida pelo OP:
well I nuked my session cache, did an explicit save from the session widget, and now it works. Well, it doesn't save the workspace location for each client and instead opens them all up on the first workspace, but I think that may be because, in the session, xfwm4 is the last thing in the "Client" list, so before it's started all the other clients just pile up in the first (and only) workspace.