Dammit, I forgot to follow this up.
Special thanks to @Armadillosoft for suggesting iNaturalist. The folks there confirmed @alexwild’s ID: they're female flower wasps of family Thynnidae, subfamily Thynninae—https://en.wikipedia.org/wiki/Thynnidae.
As @si_fuller suggested, they *were* doing something with pheromones: namely, looking for mates.
Cheers to everyone who helped out!