In theory, there are many reasonably straightforward ways to do it in software. Unfortunately, none are implemented that I know of. From my own SD DVD authoring experience I regularly used the 4:3 or 16:9 flag. What I found in practice was that many commercial DVD's either had the flag not set or set incorrectly. It still amazes me that DVD's get their trailers screwed up and have their aspects hosed. I mean if my 8 year old can see it, I'd think a person "splicing" in a trailer on a multimillion dollar DVD distribution could. But alas, often it is wrong. If you had a mdeia server where you could self-program a flag and in turn have the device control a trigger, IR send, or RS232 command send, then yes, that would automate the process except for the initial programming. But then, the trailers are still often wrong if you watch them. You also have to be concerned with DVD menus which are often 1.78 even if the movie is 2.35. If the menu selection items are off-screen in stretch mode, you have to un-do the stretch to see what you are doing, the re-stretch for the movie.
For me, the solution is to just incorporate "one-touch" macros in the remote control of your choice, with a fast transport. Then the changes can be done almost instantly with one button press, even between trailers, menus, and movies.
Even an autodetection based on video (black bar sensing) is going to get it wrong sometimes due to the way the material is put on discs.