Okay, one last update. The current SRT files for RTL languages (the ones with an -rtl extension), only really work in software that fully supports RTL and BiDi, which, as far as I can tell for media players, is none of them.
So, if you're interested in Hebrew or Arabic SRT files, I now have a new set of SRT files with a -compat extension, which should work much better. These will be released in the next version of Project Threepio, but are available now via PM.
I tested only VLC, MPC, and Notepad (Notepad is the definitive dumb text software) but all three worked, with the exception of VLC displaying a small and fairly inobtrusive garbage character on two lines in Star Wars. Other software may behave differently, because after all, we're talking about software that doesn't behave correctly to begin with. I haven't figured out a way to get rid of that garbage character in VLC without introducing worse problems, but I'm still looking into it.
So in the next release, each RTL language will have two SRT files: "-rtl", which is still used for making changes and generating graphical subtitles, and "-compat", which is created by running the "-rtl" file through a subtitle-mangling routine to create something that works better with software media players. So if a change is made to the "-rtl" file, that file needs to be re-run through the mangling routine to propagate that change to the "-compat" file.
Also, you'll need to use a subtitle editor that fully supports RTL/BiDi to edit the "-rtl" files, but finding one of those is, thankfully, a whole lot easier.