He's done lots. When I was in his shop he had dozens of slides he was working on and you can see his facebook page (MKP Arms) where he's got lots of pictures of slides he's done.
Yes he does have to mill it for a particular RMR as they all have different footprints and bolt patterns. A Burris FastFire and a Vortex Venom have the same footprint, but pretty much everything else is specific to one RMR. Glock MOS and other pistols use adapter plates to accommodate this but can create some slop in the fit.
Here's my Sig P320 he milled for me:
Yes he does have to mill it for a particular RMR as they all have different footprints and bolt patterns. A Burris FastFire and a Vortex Venom have the same footprint, but pretty much everything else is specific to one RMR. Glock MOS and other pistols use adapter plates to accommodate this but can create some slop in the fit.
Here's my Sig P320 he milled for me: