Hello,
In the meantime, I've found a workaround:
- assign a unique keyword to the portrait pictures
- search for this keyword
- select the searched pictures
- save the selection (as a text file, ok)
- go back to my (unsearched) contact sheet
- load the selection from the text file (browsing for the text file)
- invert the selection with `select others`
- yay!
Do you have better?