I don't know that you're right. I used to write scripts in Windows to control UNIX, AS400 and mainframe back ends. When I read your request, I thought, 'this should be easy' but I don't see the commands I would expect on this page.

https://usermanuals.finalemusic.com/FinaleMac/Content/Finale/FinaleScript_command_reference.htm?Highlight=finale%20script%20reference 

What I do see on that page is Press Button. I didn't drill down further to see how to specify the Rewind button but, if that's possible, then you will be able to do exactly what you want. Clicking on the Rewind Button once with your mouse backs Playback by one measure.

Yes, it should be easy, and the Rewind button, whose name should be "Rewind", should be on the Finalescript command list, but it isn't, and that is because Rewind is not (programmed to be) a Finalescript command.

I got here from a recommendation in a session with a Finale Help agent, who agrees that this is a good idea -- to have a script triggered by a keyboard shortcut that does the job of my choice of how many measures back by a single keypress.

In short, the Rewind button does work, but only by mouse, not by script.  So my message is here to encourage a Finale programmer to expand the Finalescript command vocabulary to include Rewind.

A possible work around if you are on a Mac:

Using a Mac and Keyboard Maestro to create keyboard shortcuts, here is a workaround that works for me - no mouse clicks needed:

1. Create a KM macro that maps clicking on the left/right scroll buttons at the bottom left of the Finale window to the left/right arrow keys (or whatever you like).
2. Create KM macros that map clicking the Play and Stop buttons on the playback controls window to keyboard shortcuts. Note: The spacebar start/stop won't work because using it doesn't "stop" playback, it only toggles between play and pause. For this to work, Finale needs the "stop" button pressed to recalc the leftmost measure.
3. Set your view to "Scroll View". This workaround doesn't work in page view.
4. In the playback controls window, select "From leftmost measure".
5. Use the KM macros to trigger play/stop in the playback controls window, then use the other KM macros to move forward/backward in your score, positioning your desired start measure as the left most measure in scroll view.
6. Advanced option: create an additional KM macro shortcut that when pressed selects the VIEW -> GO TO MEASURE... menu item. Then you have single bar movement with shortcuts and can also jump around in your score to specific measures.