Excellent post, and well written as always. As I mentioned in another post I've learned a lot from BNS over the years regarding boa care, and I like conversations related to the natural habitat and behaviors of snakes.
I am dealing with a similar situation when it comes to providing lighting in the cages. Like the O/P, I have the cages in a room without light, not even tiny windows. The only artificial light option is from an overhead/ceiling fixture that can't be hooked up to a timer.
I increase and decrease daylight hours and also cage temperatures to align with the seasonal changes. If your choices are limited to in cage/direct lighting, I would recommend providing hides or constructing something with the decor that will make the light in the cage less direct.
I use LED strips and set them on the low setting. The boa and royal use their hides frequently although I do catch them out more often starting around this time of year. The carpet python rarely uses a hide and the reticulated python does use a hide however he is more active and will perch mid day or sprawl out. I would really like to see the cages get true sunlight but that option isn't available here.
You can see in this photo that the hides block out the majority of the direct light. I have the lights set a bit further back to illuminate the rock backgrounds yet it leaves the front of the cage fairly dark and there is very little light in the hides. The boa is a bout 7' long and easily fits in the hides. I put in a different light in the top cage since this photo was taken. It is adjusted to be more dim. These lights also appear brighter than they are. They are slightly overexposed due to the fact the picture was taken in complete darkness.
In the evenings, I provide a few hours of LED moonlight. As I understand it, this type of light doesn't bother a snake but I could be wrong. I do catch them all out when I have the blue lights on, even the royal makes a showing. At the end of the day the cages are totally blacked out for night time.
At the end of the day, a real light cycle, with real sunlight is the way to go.