It's possible to upgrade but I think that would quite a bit work. We would need to merge M7 into our fork. I haven't tried but I think that part would be ok. The more tricky part is building library from M7 that can be used by Scala+GWT projects.
The problem with this part is reflection. GWT does not support reflection so we need to put work-arounds in place. Scala reflection has been heavily refactored and our old work-arounds will need to be updated.