Okay, so I've done about four,  low dose, week long "trials" of HRT before.  Would it be a good or bad idea to tell my therapist about this?

Good idea from every aspect. You will not lose anything.

I've found over the years that low dose HRT was a great way for helping me get over some bad humps. Sort of like a brain reset. Hormones can and do alter your thought processes and hence your emotions. That alone is a good reason for the therapist to know. I cannot think of any reason why not to tell the therapist you had or are. His biggest legit concern may be the "How?"