It currently only works with a special branch of Xwt.WPF, but I hope this can be a good demonstration of what is possible to do with Xwt and some of the changes that are needed for Xwt to be capable of completely replacing GTK+ in MonoDevelop.
It is licensed under the MIT license, like Mono.TextEditor itself.
I made this over a month ago, but I figured it wasn't doing anything on my hard drive so I decided to publish it. Even if it does not have the code quality I'd like it have, it's better than nothing.
Unfortunately, I used Visual Studio, which tends to reformat everything it can. This makes it harder to compare (diff) this to the real Mono.TextEditor, which is practically required in order to reach parity.
I restarted this project several times. I tried starting from Mono.TextEditor and moving to Xwt, and I tried starting a fresh Xwt project and importing the parts of Mono.TextEditor that were useful.
Each time I did one of those things, I learned a bit more about the original project, so each effort got farther. Eventually, I got this result (can't remember which approach I settled for). It does include a lot of Mono.TextEditor classes with virtually no changes, and it also contains a lot of completely original code.