Write some awesome patches to MIR. Refuse to sign CLA. Everybody using MIR via GPL can use the patches. Canonical can't.

This is basically what happened with OpenOffice.org. There were lots of patches that Sun did not accept, these got hosted at go-oo. Pretty much everyone (atleast all the linux distros) used the go-oo version. This was an early step in the LibreOffice fork.