I found a way to use Finalescript to select & act upon a single text box but it must be in a known place.  The files I primarily work with have the same types of text boxes & they're almost always in the same place.  But selecting them takes few tricks.

First, the view must be set to some standard setting for each file.  Then, the Ctrl + Home Key is invoked.  Next, the cursor is set to the position where the text box handle is.  Finally, the click function is used.  At this point the text box is selected & can be manipulated as desired.

Below is an example of a working script that selects a subtitle normally situated at H: = 0.00" & V: = -0.35".  The code sets the page to 75% & locks it in "home position".  Then the mouse position is set to 320 205.  These are grid coordinates determined mostly by trial & error.  Once the cursor is at that location, the click command selects the handle.  From that point on you can manipulate whatever parameters you want in the Text/Frame Attributes dialog box.  In this case it simply adjusts the vertical position -0.35 to -0.38.

Text Tool

Press Ctrl + 3

Press Ctrl + Home Key

Mouse 320 205

Click

Menu item "Text/Frame Attributes..."

Type -0.00 near "H:"

Type -0.38 near "V:"

Hit OK