apparently, you can "demux" srt subtitles to Blu-ray "sup" graphical subtitle stream, then edit it with bdsup2sub++ (c++/qt5 port of java-based bdsup2sup) and remux back into blu-ray! Or create dvd subtitles out of srt subtitles!

Or create your your own *.ass/ssa in ageasub and then use https://github.com/Masaiki/ass2bdnxml (in theory it can give you sup file directly according to built-in help)

And manipulate subtitle params with 

https://github.com/cubicibo/SUPer (uses python)

Also, not something I tried but interesting piece of software (python again) for manipulating blu-ray internals like playlist/clip info ...

Also, windows program but may work under wine?

3d (MVC) encoder (x264 can't do this)

https://sites.google.com/site/videofan3d/software/frim-encoder.

Also, is comment from https://www.cinelerra-gg.org/bugtracker/view.php?id=282#c3706 actually in manual somewhere?

I can't see it in https://cinelerra-gg.org/download/CinelerraGG_Manual/Subtitles.html but may be it resided in some other section ....

May be this advanced manipulations of Blu-ray streams also better to be put in separate html document? It really not about Cin-gg but still those pieces of info a bit hard to find so having them on our site might be attractor!