Hi Sally,
Please check out the beta version! It includes the ability to subdivide the beat, so that you can have each of the three beats of 3/4 time be a different chord if you want. (It’s got lots of other nifty editor improvements too!)
As for the hemiola (a term that I had to look up), I’m not sure if you’ll be able to accomplish that in Strum Machine yet. The subdivision may get you there, depending on what you want it to sound like. But per-measure time signatures are something I have planned for the future.
Nice to hear you like the app so much in general! 