One of the most open-ended aspects of PStill is its ability to use arbitrary control scripts (included in PStill are ones for N-up, adding a page number, tiling posters, scaling, PDF-X media boxes, etc). We've started a repository for scripts:
To select a new script: add it to your scripts folder.
To access your scripts folder, hold down OPTION while you select a script from the popup list in Preferences-> Output Options, the Scripts Popup.
This will also allow you to edit the selected script.
You may have to relaunch PStill for the new scripts to appear.