Looking into some modding research by Meph, a possibility struck me. I had long assumed that anything that was NOT_LIVING (clowns and zombies come to mind) would not be targeted by both the undead and the clowncar, which both have their hostility ensured by OPPOSED_TO_LIFE. However, it was suggested that OPPOSED_TO_LIFE will still target NOT_LIVING: it actually just marks any NO_AGING tagged creatures as friendly.
If this were the case, a method of clown disposal becomes possible; weaponizing the self-renewing resource, zombies. Simply capture a group of necromancers, give them a line of sight to some dangerous corpses, devise a means of shutting down/activating the reanimation trap (bridges and fortifications, simple enough) and send in the clowns. Clowns won't path into the necromancers unless the fortifications are underwater/magma.
Potential problems:
-Demons with ranged attacks can shoot through the fortifications to kill the necromancers within. Possible solutions: layer fortifications to make it more difficult for the shot to connect (needs testing to see how far away a necromancer needs to be to reanimate) or have multiple necromancers (if reanimated necromancers can themselves raise the dead, which isn't certain. I know ghost necromancers can.)
-Zombie Clowns. These could be better or worse than regular clowns; zombies have a hitpoint system rather than the regular bodypart system, but it's unknown if hitpoints vary between creatures or if they do what causes higher/lower values.
I can't currently test due to business, but I may in future. If anyone wants to try it before I do, go ahead. Your exploits will be told whenever dwarves compare beards.