After watching Le Mans yesterday & today I thought of this thread and this supposed mid-engined corvette mule. Looking at it again, I don't think it's a mid engined car. The focal length on the lens used to take the photographs is very long so there is almost no perspectival distortion, allowing this little rough analysis.
Assuming the wheels are about 18-20" in diameter and the door shut line is somewhat in-plane with the car's firewall (provided it's MR) it leaves around 31" between the firewall and rear axle center line. While the LS engine is overall pretty compact, the bores are still pretty large meaning the engine is pretty long. Most sources online put the LSx at 27.5" long. The Corvette transaxle is very long because it doesn't need to be compact. The input-to-drive center length is like 22" and the overall length is 31" Its unlikely it would be suitable for any sort of MR car with decent packaging. There are smaller transaxles; here is one I found on the net, a Porsche 915 unit.
This is about the most compact input-to-drive center I found, with 9" needed and the axle gearbox component behind. Still though, it seems like it would be AWFULLY tight in there. Not to mention that approximately 1/3 of the engine would be below the greenhouse part of the car.
Barring the not-so-perfect analysis above, there are other things that make me question the theory of this being a mid engined car. There do not appear to be ANY ducts for the engine and cooling system that would indicate it's mid engined. If this really is an 800hp engine, it's going to be needing a TON of CFM for the engine and cooling flow, just look at a Lamborghini Aventador or Porsche 918 for reference. And those are production cars where aesthetics rules. On a mule, I would expect massive ducts to keep the thing cool and with plenty of oxygen for the engine. There appear to be a couple ducts in the wing supports, but is that enough for an engine and cooling system? I don't know, but I would guess no.
If I were to speculate, I would say it is indeed some sort of Corvette mule, but one for R&D on a new rear suspension and/or transaxle setup. The removable panel would facilitate this. In fact, I bet this was the mule for the 8 speed automatic that recently debuted in the Z06. The timeframe makes perfect sense.