htmlmail-simplenews--[category]?

I'm trying to figure out how to assign different templates to different newsletter categories when using HTMLmail. normally I would do something like simplenews--newsletter--[machine_category]. I've tried htmlmail--simplenews--[category], as well as many other possibilites, with no luck so far. is it possible to do this while using htmlmail? yes I have cleared my cache each time.

thanks much