Does anyone know of a red dot mount that works (or can be modified to work) on this pistol?

Is it a 5in ? I'm sure there's a mount somewhere , you might check fleabay . Guzzitaco put an Sig Romeo on one of my 745's and on my 45Recon , but they don't have BoMar adjustable on them . You can check with BMCM , he's THE MAN when it comes to our S&W's .

Yes, it is the 5” version with the adjustable rear sight.