Oh my god you are my hero. This is literally and figuratively a game changer! You have opened up so many games that previously I just couldn't get into. Thank you so much!
I have tested this out on several games and so far have had complete success. I will still look to see if there is a game specific mod that does this first, but if that doesn't exist I have a perma-copy of your mod downloaded to serve as an emergency backup!
I am so bloody happy I cannot adequately describe it. Dialogue boxes have been the bane of my porn game existence for so long, so to suddenly find this by randomly scrolling some comments somewhere is just... stunning.
So far the mod has worked wonders, so no operational complaints, and I am not technical enough or Ren'Py knowledgeable enough to give you feedback on that aspect of the mod.
Holy shit, this is just... amazing. Honestly I have no idea why RenPy doesn't add the dialogue box transparency as a baseline option in the game. Your mod is proof that this is a needed feature, so thank you for finding a gap in the product and correcting it.
I will reach out if I find any bugs or note any games that it doesn't work on (unless they are the types specified in your post as not being likely to work, such as modified dialogue boxes).
I truly hope this 5 star rating others find this incredible service you have provided the community.