I don't understand what you mean by "add zthumtab ... in PA" Do you mean in the primary?
By default, the tumbtab styling is based on toc3, or if you really want to, you can (in your ptxprint-mods.tex file) override the default definition:
\def\ThumbTabStyle{toc3}%Which style defines the font? Default is toc3
Thus you can choose a new font for the thumbtab by setting it on the toc3 (or your choice) styling.
Thumbtab selection rules work like this:
* at a new file, the tab text is set to empty
* \toc3 sets the tab text IF is currently empty
* \zthumbtab sets new tab text, whether it's set or not.
Thus, to get thumbtabs defined based on the secondary text, you should be able to set it in the secondary with \zthumbtab, and a changes.txt rule can accomplish this so it's all automatic.