Hi Cindy, Hi Kelly
Oh boy! What a ridiculous situation.
It really annoys me that sound physiological principles are completely thrown out the window in favor of dogmatic teaching.
The technique described in the Hot Yoga MasterClass manual in great detail and also touched on in Opening up your hamstrings with hot yoga is absolutely the way to go.
As you BOTH and thousands of others can swear, many back, hamstring and sciatic issues are WORSENED when you lock your legs and stretch to the floor through a ROUNDED back. This is dangerous. And then to be told ‘my way or the highway’ with a risky maneuver is just irresponsible (or insert word of choice ...
).
The magical part of using the technique described (particularly for Hands to Feet and Separate Leg Intense Stretch) is that you don’t need to back off AT ALL. It simply does not matter if your legs are bent. I guarantee you that you will feel the stretch - and more importantly you will feel a safe stretch - right through the back of your body which opens up the calves, hamstrings and the whole back, while working the arm muscles.
So what do you do when your teachers say no? Unfortunately the polite person inside of us wants to please the teacher and not cause any waves. And defying them by simply not listening to their admonitions that you have to do it in the way that hurts you, can cause some internal conflict! Yikes.
You have 2 choices. One of them is to ignore your teachers and continue to do what works for you (safe in the knowledge that you are not changing the pose or defying any good solid scientific and yogic principles) and build that strong internal frame of reference that allows you to stand up for yourself even in the face of opposition. That in itself is a great thing. And I am sure you can also approach it another way.
Perhaps you can go to your teacher/s and express your feelings and needs. It is definitely hard to give you the script for how a unique conversation is going to work out. But there are definitely things you could say:
* I am feeling very upset/angry/dismayed/bewildered (or whatever your feelings are) when I am commanded to do this pose in a way that actually causes further damage to my hamstrings. I feel strain in my back and damage occurring in my hamstrings when hanging out with my hands on the floor with straight legs. I need to feel safe and that the work I am doing is benefiting me so I am requesting that you allow me to do this pose (without complaint) in the way that works.
* I enjoy coming here, and I want to let you know that I am going to continue to do that pose in the way that benefits me.
You could try showing them the post on Opening up your hamstrings with hot yoga. You can get all factual on someone but unless they are willing to listen to you and adopt a sense of curiosity, you may find you simply have to defy them in class.
My guess is that your comments will be welcomed and you will be able to do what works. You COULD get some opposition but what can they do to you? Can they force you to do it the wrong way?
The worst case scenario is that they throw you out of their studio. Yeah right! That’s not going to happen.
We all have to learn to live with feedback. And it is not just the student who has that job to learn.
I love this yoga and I really value how it has made me stronger (and millions out there). You hear feedback in the room that potentially can unbalance you, emotionally and physically - remember when you were a new student? And after a while, you hear stuff, you check in with your body and you work out whether the feedback you hear for Joe Blow on the other side of the room actually can apply to you too. You check, you apply it or you don’t. But it is you who decides. The beauty is that while you literally became unbalanced in the beginning of your practice now you just observe (listen) and you apply it if necessary without emotion.
Don’t get me wrong. I am not saying you should do your own class with your own adjustments and make up your own pose sequence (as some people have been known to do). You are not changing the pose you are doing it properly. You will feel stronger when you can assert yourself without then berating yourself or judging anyone else for the reactions it evokes or ‘causes’. When you can do that without self-judgment, others will less likely judge you! 
Namaste
Gabrielle 
PS I am overjoyed that you love your Hot Yoga MasterClass manual!!!
Thanks Cindy