The original crawl has an ABA structure and Across the Stars would work for the B section, but it would need something more energetic for the A and C sections.
JEDIT: This would have been a perfect place for a “Republic” theme which could have morphed into the Imperial March theme throughout ROTS.
Anakin’s theme, if you changed the rhythm slightly, might work. Maybe even use the variation heard in “Panaka and the Queen’s Protectors” which has the same opening phrase but with the Imperial March ending removed.