Sorry, Jim.
I thought my interpretation of the question was incongruous with your skills and knowledge.
I asked this once before and find it strange that Save Presentation returns and logs an empty string.
Many versions of JMP and of Windows OS ago Default Directory seemed to work, but with all the background stuff going on in the PC, JMP changed the behavior of Default Directory. Circa JMP 11, I argued "if the JMP Pick Directory or Save or Open has the last used path, it would be useful to expose that via JSL."
When I tried this before, I looked for a method to send a click to the Select Folder button for Pick Directory.
Something like this since the focus in the Pick Directory Window is on the button Select Folder and the path is the last path chosen.
_xx = Log Capture (Pick Directory() << Click)
But nothing tried was successful.
So a wish list item is that Save Presentation writes the file path to the variable or the log. Since it currently writes a blank string to the log and to a variable xx = (rbiv << Save Presentation() ); it does not seem like a big ask. But better yet offer a JSL function like Get Path("$Last_Path") as a session a variable.
P.S. The example application written for the book has a menu item to Save PowerPoint, so we wrote our own pick directory and file interface for the menu item then used Save Presentation(with the file name).