Feature request for something similar to the .config/xfce4/desktop/icons.screenX.rc file for Xfwm window placement overrides.
I'm unsure what the performance implications would be if Xfwm had to check newly created windows' name/title/type/class attributes against a list of user-defined placement hints but I figured I'd at least float the idea. Xfwm's placement behaviour is pretty good but there's always certain apps that behave oddly or apps where you don't want new windows appearing 'all over the screen' but in a neat stack (eg. multiple web browser windows).
Do you know about Evils Pie? It does exactly what you are requesting. It runs as a daemon and watches newly created windows to apply specific rules on it like move to workspace 2, at pos x/y.
My 2 cents
I ment Devils Pie of course, I have really a hard time getting along with this name anyway.
I didn't know about Devil's Pie, actually. That could be extremely helpful, cheers.