I tried working on this a few weeks ago but had trouble getting only onboard to load in GNOME Fallback (caribou doesn't work well at all there) but only caribou to load in GNOME Shell. In Ubuntu 11.10, both GNOME Shell & GNOME Fallback share the same xdg-desktop name "Gnome" which makes this more complicated.
Michael, thanks for you bug report.
I tried working on this a few weeks ago but had trouble getting only onboard to load in GNOME Fallback (caribou doesn't work well at all there) but only caribou to load in GNOME Shell. In Ubuntu 11.10, both GNOME Shell & GNOME Fallback share the same xdg-desktop name "Gnome" which makes this more complicated.