I do need to vary the output frequency (otherwise I'd just use piezo transducers which are driven by 12V). The frequency is important...whether it's critical or not depends on whether I ever try to do doppler anything with it.

 

There are plenty of high-voltage p-channel MOSFETs - since your currents are small, you won't worry much about Rds. One thing you need to know about this circuit - when it dies it takes everything with it; use opto-isolation at least when breadboarding the thing.