I think I figured it out. Using dconf-editor, go org > mate > desktop > session > required-components.
Once there, change the value of "panel" from "mate-panel" to "xfce4-panel"
This works only to cleanly disable mate-panel:
jake@mate ~ $ killall mate-panel
mate-panel: no process found
The change in dconf-editor will not automatically start xfce4-panel, so you have to set that auto-start yourself.