UltimateDialog.ShowAsync().ContinueWith(t => Debug.Write("returned"), guiTaskScheduler);
Or we could use a callback delegate as the parameter for ShowAsync, with another parameter being a TaskScheduler.
I suggest tasks rather than async, because that's part of the current released version, rather than a CTP.
We could provide a custom message, and a custom button list, and a custom DialogResult. We could even enable input, and even allow for a watermark.
This would be an interesting little project. Github would be a nice place for it.