Arturia would be wise to release their code to the community. Not only would the community fix all of the nasty bugs that have plagued the MIDI Control Center, but when people are empowered, they always tend to come up with the most creative ways to make use of technology.
Sadly, there is a better chance that you or I win the lottery this week than Arturia releasing the MCC code to the public at any time in the foreseeable future.
My suggestion is for you to return the KeyLab and find a Nektar Panorama - though you must do your research on this product as I have never actually seen it in action and it may very well end up being just as horrible as the KeyLab or worse. You might even be able to exchange the KeyLab for a Panorama if the retailer carries the Nektar brand. The Panorama is said to have 'deep integration' with Logic Pro X and since MainStage is built using the Logic Framework, the Panorama should also work very well with MainStage. If it works as advertised, the Panorama would likely do exactly what you need.
Again, do your research first.
Hope this helps