It can be done...
I have found a way to do this, though it involves creating an extra track for the duplicate track volume. This is done in GarageBand '08...
1) Create a new track
2) Click on the pyramid/arrow next to the track lock icon to create a new volume graph
3) Go to the region where you have created the volume levels you want to reproduce, click on the region
4) Go to the menu, Control > Lock Automation Curves to Regions
5) Option click drag (on mac) your previously created region and it's volume curves to the new track and position it where you want it to repeat.
6) Unclick the Lock Automation Curves to Regions and you can delete the duplicated music region and put what you like there, or even keep it clicked on drag it back up to where you want the curves reproduced.