I'm honestly surprised there isn't already an addon for 5th axis capability. For that matter, I'm surprised the MX machines still rely on a stepper for their 4 axis functions. Granted I don't know a thing about the pinouts on the control board so maybe there's nothing left to plug in extra servos but I would think that adding two servos to the mix wouldn't be that huge of an undertaking. Then use something like the microarc to power a trunion table with a rotating platter and bob's your uncle. There's a guy in one of the facebook groups that has more or less done that. He has a manual selector that switches between his B and C axis so it's not a true 5 axis machine, more of a 3+2 but it does the job. A little integration into the source code and factory hardware would make it a complete solution.
That said, given the price of the microarc, I would expect a setup like that to run pretty damn close to 10k which would certainly put it outside my budget for an addon. I see a lot of comments lately about the rising costs of tormach's which is certainly true but in my case, I bought a base machine with very few options and have been slowly adding to it over the past couple years. In the end I may have spent roughly what a Haas would have cost me but I did so in much more manageable chunks.
I would like to see it happen though, if only because that would open up the possibility for homebrew 5th axis mechanics tied to the built in controller. I did this for my 4th axis and while it's no microarc, it does the job for the most part and only cost me a few hundred bucks and a couple days of fiddling. My machine isn't used to make money so that was a much better option than dropping 4-5k on the factory option.