Considering that at the moment the mir dev team does not provide build from source instructions so you can build and testmir and patched-mesa together outside of an Ubuntu environment... expecting to see any external contributors is putting the cart before the horse. Mir is effectively just an internal project at the moment with no on-ramps for externals to come up to speed on Mir.
The dev team for Mir has not done the basics necessary to give externals an on-ramp to work with the sourcecode, even ahead of worrying about signing the CLA. I can't just pull Mir from bzr and test it. I need out-of-tree mesa patches as well at a minimum..and they haven't documented how to pull those from bzr or git as part of their build from source procedure. Why on earth would ANY external bother poking at project sourcecode they can't build up and test from source? That's just crazy.
This lack of information doesn't reach the level of "hostile" .. but it certainly indicative of inexperience actually knowing how to be a viable upstream project, instead of a distribution-specific team.