Diw, this is a good question. Of course it is possible to do the MC mod if requested, but the majority of the Audiophiles sitll go for Stereo and this might be the reason why MC mods are not widely offered. Personally, I do not like MC. I am a guy who wants to sit on the first row and enjoy. Imagine 10 feet deep, 6 feet tall and 20 feet wide holographic sound image projected infront of you that expands far away from the outer side of your speakers. That is all I care about.
As for the MC mod, you can have it anytime you want. Of course, since there is no space for 3 tubes and 6 output transformers, the mod will be all Solid State with MOSFET analog output stage. The "full monty" would involve 3 DAC boards, 6 coupling transformers and 6 MOSFET output stages. As you can imagine, it gets really pricey, even without tubes.