I have had this thought for a while, but it's just a thought as I do not have the facilities (or contacts) to build anything like this. But sitting way too long with Foxdie yesterday, somehow it came up, he thought I wasn't stupid, so, here:
One thing that bugs me about RDS on a pistol is how much, and in which dimension, it changes the package size. It even causes issues for holstering, etc. But there are other spaces on the gun where adding stuff wouldn't matter as much.
So that and other ideas make me think, what about an occluded sight on the BACK of the slide. You could mount it much like this:
Here I have postulated an RMR-like sight to the rear, retained by the rear striker plate mounting bits, AND the rear sight dovetail, all used to retain the unit. Without actually measuring too much I also have distributed the bits to keep size down so that the battery is in the lump forward of the rear sights for example. The optics could even intrude into the gun by using some of the volume of the striker plate, etc.
The sight picture is then like this:
Or of course, occluded. You open both eyes to use your brain as the collimator, and get a red dot on the target just like a windowed RDS.
Yes, there are (probably normal height) irons above for backup, and if you could arrange it as such, you could get the "sight window" high enough to get the dot to be placed very high, not much below the iron sights. Which may be helpful.
It might then have parallax differences between center of window, but who cares if you zero for your preferred sight position.
I would try it myself, but think this would be hard to back yard gunsmith on. While an RMR could certainly be mounted here, it would be an awful big mount by the time it got solidly attached. So sadly, this needs a complete design by someone reputable.
Still 1) we did crazier ideas in the first few pages of the thread and 2) crazy ideas come to market!. After playing with one, I have ordered a Bullseye FT to try on the Kahr (too small for an RDS IMO, anyway).