I imagine if there's even a possibility, then a company like OWC will do it. They've been pretty good at supplying upgrades for MacBook Airs and keeping old Mac Pros going too.
I think you are grossly underestimating the complexity present and the skillset required to pull this off. Apple's slightly less wide and longer mSATA SSD drive is still primarily still just a simple SSD drive. There are no cooling requirements out then to let the rest of the mac blow cool air on the SSD. Maybe Apple has a extra pin on SSD connector to monitor metadata but it it all pretty much mainstream SSD.
This is a graphics card unlike anything on the market.
It has no fans or self contained thermal management system.
It has no standard ports of any kind on it.
It has a PCI-e SSD card on the back.
It has a connector that has an extreme likelihood of being custom design explicitly just for this Mac Pro's needs ( not unlike the CPU/RAM daughter-card in the current Mac Pro because it is effectively plays
EXACTLY the same role. A daughter-card part of a logical mainboard with multiple physical subsections. ) There is nothing to indicate that Apple is following any standard.
That is not in the "stretched out", standard SSD zone.
This card has to:
Seamlessly integrate back into the fixed thermal management system. That likely means very specific physical tolerances. ( that is possible but not necessarily easy )
The foreign GPU+infrastructure design needs to exactly match up to what the Mac Pro can profile cooling for. No one has a reference implementation design for this. Apple has mutated AMD's FirePro but AMD doesn't particularly hand out reference designs for FirePro cards. AMD only directly makes (well contract makes like Apple) those themselves. So probably talking about mutating a reference design card that hasn't been designed for this Mac Pro.
Need to reverse engineer the custom connectors.
Apple is probably going to have a zero tolerance warranty impact policy on non-Apple daughterboard designs. Storage drives based on standards is one thing. Completely custom Apple parts.... no there isn't going to be any certification process for that. So the target market is customers looking to toss their warranty out the window.
It would be extremely more commercially productive for OWC for focus on the PCI-e SSD card. Apple has likely used for more standard parts. That will lead to a far more higher ROI. The storage card like the RAM has all the appearances of being positioned for the intention of removal being an occasional user action.
It isn't impossible to make a card. However, with all that custom work are you going to have the volume to get to a price point that is lower than Apple's with some better performance metric. Are there drivers for the GPU?
It's a big enough market for a small company to do it, if not Apple or the GPU manufacturers themselves.
The 3rd party GPU makes barely want to get into the Mac Pro market when it only meant using a standard Nvidia/AMD reference design and doing the differential driver work. Rejiggering a whole card to fit Apple's thermal system as opposed to one of their choosing represents even more risk with questionable reward.
AMD has little interest in messing with the profit margin "goose the laid the gold egg" in lowering FirePro prices. That Apple has completely physically segmented their cards from these is a good thing. Nivdia... again no. If Apple is signing up to do all the board work .... why? If they win the bake off for the next round Mac Pro Apple will just buy the parts and do all the work plus pay a fee to get access to the reference design. There is little to no upside in poking at stick in Apple's eye if they aren't interested in 3rd party cards for the Mac Pro. That just makes it more likey will loose the next design bake-off for Mac line on the next iteration.
As I said there will probably be a bone-yard market evenutally where some dead Mac Pro with OK high end cards drift down to Mac Pros of same design that had lower end version. That will allow a subset of owners to add another year or to to their 3-5 year old machine.