Ah, now that is an interesting question. Whilst I agree that the extra menu would be in some ways a neat solution, it does make the experience a little more clunky.
One way might be to have your suggestion in the bar above once the movie is selected (in theatreview). Currently when you select a film from your library by clicking on the image, it takes you to another screen which displays a bigger version of the movie art and the movie description text. At the top of the screen there are several options, I.e. "Back", "Watch", "Tag" etc. How about inserting another option next to "Watch" called "Watch (external player)"? Just a thought as it wouldn't change the flow and keep it a "family friendly" experience.
My current solution is as follows: I've set up a new option on the main screen under "Video". Next to the "Movies" category I have created a new one called "PowerDVD". For every blu-ray in my collection I have created a "particle", and each particle I have tagged as an "OGV" Ogg video (as I have no OGV and never will). Under file types I have designated these to be played on PDVD via your script. The PowerDVD menu item I created only shows the OGV particles, so selecting any movie in the PowerDVD section will automatically your script. Bit of a pain to set up but once done it actually provides quite a smooth solution.
Hope that makes sense.