Specify a custom "Movie rec. size" on Canon Rebel
Short answer: no. The presets you get are what you are stuck with.

If you can do 1920x1080 at 30fps, it's easy enough to use a video editor or converter to go down to 1280x720 with minimal loss.

I frequently would record at 1080p on a Canon DSLR, and then finish and render at 720p (with the same framerate).