Comment 9 for bug 1542028

Forgot to mention; you set the title using the -title <string> option. By default it's a fixed string of your choice, but an ampersand tells Xmir to try and guess the window title from the window tree:
   Xmir -title @

It's not enabled by default because it only applies to running single window (non-rootless) Xmir with a single X client, itself in a window.