I'm attempting to keep my reply brief here, Tim, but this is a topic that I'm extremely passionate about. It would be an outstanding addition to have the 'Factors transmit their internal controllers over MIDI. As for the ROI for Eventide … I'm not certain that the demand for such features would balance out the R&D costs involved. Anyone else interested here?
I hadn't considered the Moog CP-251 … until now. It doesn't have MIDI output, but the (recent?) addition of CV attenuators now has me intrigued. If one could limit the CV ranges to 0-3V, there are possibilities with hooking it up to a 'Factor expression pedal input (see UM). I'd be wary of this until I found out a little more about the 'Factor circuitry (potential damage with higher voltages; current limiting). I share some of the same concerns with the MP-201. That's a heathy chunk of change for a control pedal. While the LFOs transmit over MIDI, and they're fairly configurable, I would pick it up if I was sure that I could make use of the envelopes as well (CV output only).
I'm currently using the Lexicon MPX-1 in my rack; probably more often as a MIDI generator than the onboard effects themselves. For a peek at the power & potential, check out Section 8 – MPX Internal Controllers (p. 103) of the online manual. Two LFOs, two envelope followers, two ADSRs, an arpeggiator (sending notes/CCs), triggered A/B "glide", random, sample & hold, and a host of other controls that transmit, process, "munge", and merge MIDI (including all CC messages).
That rackmount device adds 'features' to my 'Factor, ADR III, Whammy IV, etc. that wouldn't be possible otherwise; all synchronized by the MIDI clock. I'm trying to work in a hardware sequencer (transmitting sequences of CC values in sync). That brings the studio to the road. Right now, I'm working with some very old Roland and Korg sequencers. Newer hardware sequencers (those capable of storing CC sequences) are either prohibitively expensive, laden with features I don't need, cheaply constructed, or nonexistent. I just might have to follow through on building one someday (that's 'bulletproof').
For what it's worth, I'm in the process of working out the CC values for ALL of the PitchFactor's parameters and settings, and I hope to publish the .pdfs and MIDI files on this forum when it's been completed & tested. I need a complete reference guide for what I do with the pedal, and perhaps that would be useful to others, too. Ideally, I could adapt the information for the other 'Factor pedals.
I'm not so sure about lugging around a theremin just yet, but I have looked into D Beam-type devices, or an onboard guitar device (based on the Doepfer / Wheel Electronics modules). I would need something easily accessible, yet small enough device to 'stay out of the way'. Speaking of which … I also use the Behringer (!) FCB-1010 to send program changes to five devices, and discrete levels of CCs to the PitchFactor. It might've been the best $150.00 US that I've ever spent.
It's huge (which is both an advantage and a disadvantage). The price is right, the build quality is unexpectedly good, and the manual is horrible (as is the programming method). But the features are deep, and there is a freebie editor that helps to lessen the pain. One-touch access to 10 presets at a time, the multiple program changes, plus two expression pedals, and an ability to mimic the high/low limits of an aux switch on consecutive foot presses.
For example, I have a few 'banks' of ten presets each that are intended to control various aspects of a single Pitchfactor preset. Interesting topic, Tim, and one that can generate a lot of discussion (as you can well see). Your move … 
