Yes, an obvious best guess would be the BCF midi control via X32 ethernet remote port. This should leave 2 pairs and a possible shield available for the talkback mic and cans amp. Russel has previously posted of preference/desire for tactile control.
I personally prefer WiFi remote mix controlfor most times and situations, but fully understand and appreciate any one`s desire or need for tactile control.
If into sharing, I thought there may be a possible pic, sketch, report from the field for the asking. One less cat5/6 cable run for an InstaSnake & X32 remote ethernet port FOH deployment piqued my curiosity and could warrant consideration for particular situations if practical.
Your guess is pretty spot-on. The adapters aren't anything photo-worthy (they're literally built out of $20 worth of parts from Home Depot), but it does exactly what you're suggesting: 100Base-TX Ethernet and 2 XLR connectors (one male, one female) goes in, and a single 8P8C jack comes out with Ethernet on pins 1,2,3, and 6, and analogue audio on pins 4,5,7, and 8. Plug in a short patch cable and an EtherCon coupler, and it's almost road-worthy (and lets me use the hundreds of feet of shielded cat5e cable with EtherCons I built for AES50 snakes).
The RPi's software was designed to work with the v1 firmware the X32 Rack shipped with, so instead of simply shunting MIDI down the line (the original X32 MIDI specification was... well, it wasn't, really) it does bi-directional Open Sound Control translation. I don't see any reason it couldn't be done with X-Touch-style MIDI-over-Ethernet instead, but because of other projects I know the X32's OSC implementation pretty well, and I've found it to be thoroughly (if unofficially) documented. (Incidentally, I don't see any reason why my adapters wouldn't work with one of the X-Touch units in place of the BCF2000 and the RPi, although with a used BCF2000 going for very reasonable sums on eBay, my solution is almost certainly more cost-effective.)
A lot of the sound work I've been doing for the past couple of years is in the realm of vocal jazz or vocal a cappella, where I find I spend much of the show making constant, small adjustments to a lot of different channels, and where having tactile control surface (with tactile solo buttons) is almost a can't-live-without-it. Different situations call for different tools, though, and I probably wouldn't have bothered with all this if I was doing more traditional rock and roll. To be clear-though, a tablet is absolutely vital to this rig, as my BCF2000 is only setup for main LR channel faders (originally only DCAs), mutes, solos, and a couple of function buttons (mute groups, tap delay, talkback). Everything else (EQ, dynamics, aux mixes, etc.) has to be done from the tablet (which, incidentally, is why bi-directional translation for the BCF2000 is nice: I can wander around with the tablet during sound check, and the control surface will always be in sync with that I'm doing; it's also a neat party trick to demonstrate to the uninitiated
As for a report from the field, I've been using this rig for the past year or two, and it's the only mixer that I do one of my more popular clients on. They often get hired to do events where they're only doing sometimes as little as one song, and the ability to roll up with their IEM mixes (and more esoteric EQs for beat-boxing, etc.) setup and ready to go means a half-hour sound check becomes a possibility. It's definitely done a couple of shows with 500+ people in the house (and recently in a packed 1700 seat venue).
The only time I've ever run into trouble with it was a somewhat cobbled-together outdoor show where the BCF2000 wouldn't respond to any control signals from the X32, although mercifully it was still sending messages out. Of course, when I got home, I tested the system and everything worked fine again (and has worked fine since), so I'm operating under the assumption that the mains power at FOH (which consisted of a lot of crappy orange extension cables) was so under-volted that the BCF2000 wasn't getting enough power (which makes me thankful I wasn't trying to run an actual mixer out there!). Even so, the show came off just fine and, as I say, no issues when the BCF2000 is less than 200' from the nearest outlet and not sharing power with 12 amps worth of powered speaker.