Fontforge is a bit of a mystery I'm afraid
You might want to look at the
fonttools package, if all you want to do is change the PANOSE data. You can either use the ttx tool, or figure out how to use the Python library.
Hint, you want the 'OS/2' table. That should contain both the PANOSE data, and the fsSelection (AKA font forge's stylemap) field.
The idea with the ttx tool is it converts the font to an xml representation, you edit the tables in a text editor, then it recreates the font from the xml when you are done.