For anyone who finds this thread via a search engine or otherwise, since posting the above I've bought a paid subscription to Jio Cinema and can provide some new information.
Firstly, StreamFab does work with it, but with a number of limitations. The main thing is that the initial analysis to find seasons, episodes and so on completely fails. Funnily enough the error returned is a yt-dlp error, it's a very simple one to fix but it requires StreamFab to be updated to pass the correct parameters to yt-dlp.
Now in spite of this failure, if you open a title in the browser in StreamFab, the DRM MPD downloader successfully sees the title, and you can download it then with just one caveat: no subtitles. This in spite of the fact that subtitles are definitely available for all titles I've seen, i.e. HBO shows.
An interesting thing is that if you're prepared to put in the effort and have some programming skills, there are free tools on the internet which let you download from Jio with a lot of manual work. I would prefer it if StreamFab could build something into their software as it would make it so much easier just to queue up shows as with Netflix and Amazon downloaders, but that functionality isn't there at the moment.
Also in case you are interested, a tonne of titles on Jio aren't DRM-encrypted at all, so can be downloaded with various free tools. It seems to be just the HBO and Paramount+ titles that are protected.
One more thing to be aware of: it seems that the best audio codec available, at least to download, is 93.4kb/s 48.0 khz, AAC, which honestly isn't that great, but if that doesn't bother you then go ahead.