A Mark V short head would be a dream -- i.e. can be racked in a standard 19" rack like the Mark IV, but alas... won't happen. My friends at Mesa informed me at least a year ago that as much as they would love to offer that, the new board in a Mark V and related components can't fit into a 19" wide enclosure, so it would be a major redesign that requires a new physical board inside.
It is very easy to order custom rack cases that have a space for a Mark V head in the bottom and then 19" rack spaces above it.
If you want great MIDI control over your Mark V -- everything you can do with the footswitch, at least, RJM Music Technology has the RG-16 and Amp Gizmo that will give you simple MIDI control over everything.
Triaxis... I would LOVE to see a new Triaxis II that contains the preamp section from the Road King II instead of the Mark-series preamp circuits. A new design is in order, though, as the interface for scrolling through the various settings and loop, etc. is a pain in the ass. I used my 2:Ninety power amp with the RJM RG-16 and this gave me instant single-button access to selecting Loop settings, and Modern or Half Power settings, etc. instead of the ridiculous way you have to scroll through the options on the Triaxis.
Interface is old on the Triaxis rig, but hey, at least it sounds great overall
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Scott