This should be a pretty straightforward change.
We now use Ævar's MakeMaker::Awesome to get a reliable way to muck with the Makefile.PL contents. Then we add a "add Mouse 0.40 only if Moose not loadable."
No hassle for anyone but me, I think! :)
do not require Mouse if we have Moose
oh, document that we need MM::Awesome