From the IQ250 testing I've seen which has the same type of sensor technology but a slightly larger pixel pitch (5.3um vs 4.9 on the A7r) I would not be too optimistic about technical wide angles, even the retrofocus Rodenstock ones (which still are much less retrofocus than a typical wide for a mirrorless camera). It will probably perform slightly worse than the IQ250 due to the smaller pixel pitch (unless it's matched with a shallower pixel depth, but I doubt that).
It will sort of work, the huge dynamic range will take care of color casts in a good way, but you'll have quite severe issues with crosstalk meaning desaturated/shifted colors and difficulty to demosaic well for most raw converters due to the crosstalk messed up green channel.
Like the IQ250, the A7r sensor has microlens offset towards the sides, which is designed to better handle wide angles -- but only when they are not shifted. For large shifts the microlens offset will have the opposite effect and cause a sudden drastic loss of performance. You'll also see a large variation in behavior if you have the camera in portrait or landscape orientation, as due to the wiring on the sensor the crosstalk is different horizontally vs vertically.
So I'd say that like with the IQ250 you probably would see surprisingly nice initial results at first, but when you really analyze the results you see a sensor pushed past its limitations, and personally I much rather would use a fuzzier standard retrofocus wide angle lens (like the Canon TS-Es), than put on a tech wide angle and run with these issues.
Still I'm very curious to see what it can do. My guess is that you need to get up to as much as 50mm focal length of the tech cam lenses before the sensor starts to behave well.
A sensor is designed with a specific "critical crosstalk angle", and a lens is designed to not deliver light over a specific angle, if those are compatible it will work well. Unfortunately these numbers are not easy to get from either sensors or lenses, otherwise we could see directly on paper what would work flawless and not. In practical tech cam photography one usually exceed the critical angle a bit though, as the image quality degradation is generally not too drastic for mild crosstalk. You get some crosstalk on a IQ260 with a shifted Rodenstock 40mm too... so it will be a bit subjective what works and not.