SRT format has no layout or orientation data in it, so vertical subtitles are possible, but would depend entirely on the playback software. I don’t know of any software that does this, and I don’t see any way that it would be able to distinguish between the subtitles for onscreen text and dialogue. i.e. even if the software could do this, all subtitles would be vertical, or none.

PGS subtitles are another matter entirely, and I looked into this many times over the years. Back when I started this project, using EasySUP, vertical text wasn’t possible with that software. Later, using Pango, vertical text either wasn’t possible, or wasn’t possible on Windows (I forget exactly what the limitation was). Now that I’m using HTML, vertical text is easily doable using CSS writing-mode: vertical-rl. I thought I was about to be able to do something cool.

But now that it was possible, another problem revealed itself. Unlike the pan-and-scan versions of Star Wars above, 2.39:1 presentations simply don’t have very much vertical space to work with, if you want your subtitles to be CIH-safe (i.e. to display only within the video frame). Sure, if you knew that you would be projecting it on a huge theater screen, you could reduce the font to a tiny size and make it fit – but for something that would be legible on a standard TV screen – no, you just couldn’t make it fit.

So, in a nutshell, yes, it’s possible, but no, I no longer have any plans to do it. You’d have to extend the subtitles into the black bars, which would get cut off on some projection systems, or shrink the text so small that it would only work on a huge projection screen. Since my PGS subtitles are designed for playback on an unknown variety of systems, I can’t risk changes that would break things for anyone. However, if you want to do this yourself, it’s technically possible, if you want to make your own PGS subtitles and don’t mind writing the code to make it happen.