Using XNViewMP 0.5 x64
First change Browser - File list - Custom Filter so that Video is opened with associated program.
Now when you double click a video file in the browser it opens in the associated program, just like you would expect.
Then set double click under Keyboard/Mouse - Switching mode to either "Browser <> Fullscreen | Viewer <> Fullscreen" or "Browser > Fullscreen > Viewer".
Now when you double click a video file it is opened in fullscreen mode in XNViewMP and ignores that you want it opened with the associated program.