Hello Hawg, ,,deda,, here and I agree with K Bartos on the cableing. I think for sure thats cat-5 which is what Furman uses for all of their headphone system remotes, copy and paste this "
Furman HDS6/HR6 Headphone Distribution System from zZounds.com! " to see the Furman mid-range system thats damn cool in my book for the money. I have 2 of these systems and every one in the live room has their own mixer to tweak. The cat-5 (ethernet cable) is daisy chained between the remotes so its pretty simple. UNLESS you only have 1 cat-5, the Furman system needs 2 which comes with the remotes 1 grey and 1 blue, color is not a issue, cat-5 is cat-5. The photo appears to have another cat-5 that is near the pvc pipe running behind and hidden by the insulation then about 1" reappering at the lower right hand corner of the cut pine, yes this is a pine board, but you can't see the end of that cable. My question to you is are there 2 cat-5 cables? If so I bet its from a Furman HDS6/HR6 Headphone monitoring system. You can add a breakout box and run numerous remotes each with 2 headphone jacks. You can get these systems on Ebay and I've seen them on Gearslutz as well and very reasonable. If you only have one you can ducktape to your single and pull a pair through the chase, maybe meaning if theres enough room, but my guess is theres 2 already.
As to your second question theres a lot of vendors that will make custom XLR panels for you with a snake of your length and choice of any number of termanations, trs for patchbay, bare wires for soldering or punchblock patchbays, XLR to console or micpreamps, Elco or 25 pin D-sub connecters(8 channel) or even have them install XLR females on the live room side, determine the wall thickness and install XLR males on the control room side. My point being its not just a XLR wall box your needing, you also have a HUGE decision on how many XLR's you want, how long of a snake you need and what types of termination's you'll need in the control room. If you are into soldering you'll save a lot of money by building your own. You can buy blank panels and install your own XLR's and some TRS's and a Furman headphone distribution center. I have somewhere a couple of XLR panels with XLR's installed that came from a Neve setup I'll see if I can find them, harder than it sounds, I've been here 18 years and have stuff I need all the time but can't find it due to my packrat ways and 8 & 1/2 acres of barns and other outbuildings but I will look.
One more question what is the PVC pipe and where does it go? I'm guessing it is a chase for wiring and has nothing to do with the plumbing. I'm sure it's not because sewer gas would already be a problem. Hopes this helps and if want pm me your email and we'll chat off the forum, regards ,,deda,,