Has anyone come up with a workaround that enables them to test a multi-threaded strategy?

Here's what I came across. As I offload calculations to other threads and don't block in the onBar/onTick methods, the historical tester keeps running full speed. By the time the calculation is finished, the historical tester is far ahead of the data that was calculated. So any decision to open/close a position comes late. The symptom is that each run on the historical tester yields different results with the same entry/exit conditions as different threads finish calculating at different times each run.

Is there any workaround for this?

You can call IEngine.getType and if it return TEST then run your jobs in the same strategy thread

I did understand that. I suppose it could be done if the main thread had to wait for all worker threads to finish before leaving onBar/onTick as well... which would enable multi-threading during testing?

Yes, that's correct. If you do multiple tasks in parallel then it will be multi-threading, but if the strategy thread will wait for the only one thread, then it's sequential execution in multiple thread, no parallel execution in this case.