Is hot yoga safe for people with lower back pain?

I’m not sure what you mean by “hot” yoga. Is that more of the astanga (sp?) type? Movement, especially loaded, certainly adds stress to the lower back. Stretching after the workout is best, the soft tissues are warmed-up. Typically I recommend supine single-knee, single-knee, double-knee. As far as progression, more sets and more reps.

take care.