Check out hp-tactical.com. No idea of back log but they can do it. Guy at the range has a beautiful Kimber with a Docter milled into slide done by them.
"...a qualified machinist..." All gunsmiths are machinists, but not all machinists are gunsmiths.
This guy, in California, starts at $600 for the installation. His site has some pictures of his work on a Sig 226 too. http://www.tjscustomgunworks.com/
I'd contact Sig USA and ask 'em. https://www.sigsauer.com/company/contact/
BTW, called sig, the new romeo 1 will not be shipping until summer. Some dealers may have some before that.
They will not recommend a shop to modify the gun.
They may have a slide with the romeo 1 at the end of the year.
Give Greg Derr a call, he might be willing to take care of it for you. I certainly wouldn't just send it to just anyone with a milling machine that is a "smith".
Make sure whomever you choose they understand the proper angle the sight should be mounted at, if they don't you'll be wasting a lot of time hunting for the red dot.