Jonzinator, The good news is: Yes, this is possible!
What you need to do is,
0) Prepare the IPTC stationary pad the way you want it to be
1) In the navigator, right click on the main folder and in the pop-up menu, select either "Open Folder and Subfolders in new Contact Sheet" or "Open Folder and Subfolders replacing Contact Sheet".
Note: this only works if non of the (sub)folders are currently in any of the contact sheets.
2) Select all image files
3) Apply the IPTC stationary pad to the selected files
The bad news is: with 20.000 images in one contact sheet, it is going to be SLOW. In fact I'm not even sure PM can handle this many files in one sheet (try it and let us know
)
Because of this, you may want to split your action into smaller groups of (sub)folders
Good luck and patience