While I would say that anyone with low back issues that is cleared for exercise can participate in most exercise class if they follow proper progression and form and that I feel yoga is a viable movement and balance system for this group if done with caution (and again proper progression, etc.), why do you need to add the element of a heated environment? What physiological response are you attempting to improve?
I don’t see how this is viewed as a good thing. Why not add high temperatures to all forms of exercise then? Did none of these instructors pay attention to the heat stress segment of First Aid? It is a fad, not a legitimate enhancement of yoga. Sure if you live in a place where it is always hot, you need to exercise and it may not be possible to have a cool place in which to do your exercise. But in this scenario you need to reduce intensity and duration accordingly. Then progress either parameter appropriately.
I would recommend the careful addition of standard yoga for such a client. If they enjoy it and are benefiting from yoga, why are you going to add the element of heat? If you want to do this, you need a very sound answer for “why?” you are doing it. Not just to do something new or popular.