I've tried exactly what you're talking about, and, in my experience, it doesn't work. I'm fairly certain that the engine will still down-convert them to 640x272, then back up to your native res. Try converting one of the high-quality movies to 640x272 and compare how it looks and plays in-game to the same movie up-converted to your native res. In my case, the down-scaled movie looked better and played more smoothly than the up-scaled one, even though the latter had been up-converted to my native res. It also looked and sounded better than the movies that shipped with the game.
Even the hex-monkeys over at the Widescreen Gaming Forum never figured out how to get the engine to properly scale the high-quality movies, so we're pretty much stuck with 640x272. I've never had the opportunity to personally test the high-quality movies on a monitor with a vertical resolution of 1200 or higher, so I don't know if they'll work properly in that case or not.
If you come to the same conclusion I did, just convert them all to 640x272 and replace the originals with them. You'll still end up with better movies in the end.
EDIT: You've figured out how to batch-convert with RVT? Would you mind telling me how?
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