@felt, Yeah I would definitely say I am the poster child here for bad backs. I read all your guys posts and let me give you my 2 cents and some may seem like I am talking down, it is just me trying to figure out things as I write them so bare with me.
In my case my lower back issues and even thoracic issue has nothing to do with muscle issues. The muscles are simply reacting to injuries in my lower back. I always say my body is smarter than me. My body knows there are injuries in my L5/S1 and my T5. The muscles tighten to try and provide stability. The more injuries,the more you push it, the more it till tighten! Go too far the muscles lock up stopping you from moving. I am happy to say even with all my issues my back never totally locked. It is god awful pain.
There is a difference between muscle pain and nerve pain. In my case, most of my pain was nerve. The nerve pain is causes by in lower back issues from pressure on the sciatica nerve. It is one of the longest nerves in your body. The more pressure on the nerve, the further down your legs it will go. Again muscles try and hold the spinal column steady so will not press on nerve but needs help from core muscles to provide the most stability, similar to doing a squat or deadlift. In many cases that helps but still may not be enough.
Also of note the spinal column discs all work together so problems in L5 will put more pressure on L4 which usually leads to problems in L4. That is why surgery is avoided at all costs cause in surgery they fuse the bones which means less movement and more chance of problems above and below the discs fused.
Now all this behind us, my advice to you felt is to try and find a mckenzie certified PT person in your area. I believe for most bike issues it is not muscles but pressure on the L5/S1. They can show you exercises and perform a different set of spinal manipulation that is not like chiroprators, very different. The exercises or stretches I do go in the exact opposite direction. They are the cobra and mini-cobra as I am trying to get pressure off the nerve. In bending forward and sitting, you are putting pressure on the nerve in my case. I do exercises like the begging man to stretch in the other direction but would not go as deep down as you are doing in the exercise. Since I know my issue is nerve I would use a heating pad to loosen the muscles before I go to bed. If I feel pain going down the leg I would use ice to calm down the nerve. Most recommend combinations of ice/heat on/off 15 minutes at a time.
BTW, I did do your exercise for the heck of it and it felt good but not through the spine, mostly glutes as an FYI. As far as stretching goes I am not the biggest stretcher and there are many idea of what and how much stretching to do. I know guys who were hard core stretchers and would stretch for one or two entire workouts a week but this was not easy type stretching. It was nutty. I lean toward danfoz that some stretching is a good idea but I tend to lean on the side of less/easier. Most folks will tell you with back issues posture is more important.
Now for the fun stuff, cause of Saturday I did only 2 hours on Sunday and a 120 TSS. Was off yesterday and today was ready for the 3 hour stint. BTW you hit it on the nose, I now aim my ride for a TSS score but it has to be made up of a certain zone, today was >150 and mostly L3 with some L4. The strange thing was today it felt so easy to do and nailed a 217 TSS without feeling dead and the third hour again being a high. I even nailed again a 230 at the end. This was one of my best rides this year on the trainer and one of the easier ones. I can really say looking at the way I was plowing through hour 3 that my ftp is higher than 245 now and I can in the next couple month or so pull off a >210 for 3 hours. Still doing about 200-210 now. I am going to up the numbers a bit more Saturday. Tomorrow will be L2/L3 and TSS 130-150.

On a different note, my back did well. 3 hours and not too bad. I am sure tomorrow no matter the zone my back will be more sore to put in that much time on the saddle on the trainer.
-js