I've been digging into it a bit myself, but as far as I can tell a mod isn't able to update the font files, though I'd be be very happy to be proven otherwise! My hope is for a mod in this space to extend that support, but to do so I think it'll need three things:
- The files which need to be replaced and instructions on how to do that
- A core mod which updates the vanilla text with the expanded characters and has something for mods to point to
- Code which mods can include to automatically strip the special characters out if they're not running the extended character mod
The first one is a bit of manual faff which would be great to avoid if we can figure out a way, but even then I think the third bit is the most important as otherwise it makes the barrier for third party support a lot higher as people would essentially need two full sets of text for their mods. Unfortunately my attempts here have also come up empty so far.