Hi Lara,
That's the first time I have ever heard anyone raise a concern about that. Actually I would have imagined that HRT would be beneficial, especially if Finasteride is included in the mix.
I have been on HRT for several years, including Finasteride, and hair loss stopped pretty well immediately afterwards. Based on this I really don't see how it could have a negative impact on transplants. BTW, treatment with Finasteride is always recommended prior to hair transplants.
Last detail, I did extensive hair transplants just before Christmas and it has worked out really well with minimal, if any, loss of grafts. Regrowth set in after about 2.5 months and coverage is already very good.
Wishing you all the best.
Donna