Changing your aperture (using Tv) is going to create a noticable change in the image as the depth of field and sharpness change. I realize that you're focusing at infinity, but I think you'll still see the shift. Not to mention that most lenses produce unacceptably soft images when fully stopped down (we were talking sunrise, not sunset, correct?), IMHO. And I think you'd be relying on chance to have the smallest aperture limit the exposure just when you needed it.
I would try (and perhaps I'll have to give it a go) Av and use exposure compensation gradually. Then I'd use a program like the one I listed above to smooth out the EVs. It seems this process would work much better for sunrise than sunset as you can choose your exposure in the darkness, which is the tough part. So long as the camera doesn't overexpose the light side shouldn't pose a problem, and it's easy to underexpose in post.
Edit: Found this interesting page. Note the section on the bottom. Also note, that they call sunrise/sunset timelapses the "holy grail" of timelapse, alluding to you not being the first person to seek this quest and the perfect solution hasn't been found yet. Though this page is old, and I suspect the solution lies in smarter software to organize the data for you.
http://forum.timescapes.org/phpBB3/viewtopic.php?f=4&t=1871
Edit Edit: There's a link at the bottom of that link that goes to another interesting page, where they cover several different methods in detail with some great videos. One of the primary is to use Av, and ramp your exposure, then fine tune in post. Another being to use a set exposure and do it all in post.
