There are multiple panelizers available for KiCad, but only one is suitable for easily placing multiple oddly-shaped PCBs onto a panel: the kicad-util panelizer.
You simply place the PCBs next to each other and draw lines between them on the Eco1.User layer. Then run a kicad-util command, and voila! The lines are transformed into mouse-bites.
Unfortunately, kicad-util is not compatible with KiCad 6’s updated PCB format, so the drawn lines are no longer converted to mouse bites.
Luckily, a merge request contains the fix, but kicad-util’s creator is not available anymore. Although the contributor of the merge request created his fork, the .jar file has not been recompiled, so the fixed version is not easily accessible to most users.
Given the above, I recompiled the fixed kicad-util version with the mvn package command and made the .jar file available. I use it as:
java -jar ~/bin/kicadutil.jar pcb -f=myboard.kicad_pcb panel --inset=0 --hole=0.35 --pitch=0.7 --width=2.5 --fillet=1