Hi George,
You are living on the edge! Wait that is Rock 'n Roller Coaster featuring Aerosmith's ride and not Tower of Terror!
I applaud your creativity with proposing to your girlfriend at Disney's Hollywood Studios. I think it is a wonderful place to start your future together as a family.
The trick with Tower of Terror is that there are random drops therefore random times for the picture. It usually is on the first drop down but it changes and I do not want you to miss the moment. Also, I would want your girlfriend to hear the proposal and the ride is full of screaming guests including myself when I ride!
If Tower of Terror is her favorite ride, perhaps you can have the cast member at the entrance of the ride help you out. Or even the bellman at the elevator. I would reach out to the wedding planning team at Walt Disney World for their advice as they live for this type of magic.
If you do go with your original plan, tie that ring tightly to your body or it could vanish into the Twilight Zone!
Congratulations and wishing you a magical vacation!