Don't give up just yet. Though it seems like a huge challenge, I'd love for people to try and take it on!
As a basis for the plugin, the source code for context-n can be copied: https://github.com/adlez27/context-n/blob/master/context-n.py
Dictionaries could exist in correspondence between specific reclist formats (or to the individual voicebank, in the case of context-n's config files) and a universal encoding internal to the plugin: ideally narrow-transcription IPA for the greatest amount of specificity.
Every plugin-internal phoneme could have its own sub-list of every other phoneme in decreasing order of similarity, for substitution purposes.
Users would specify the source encoding, and the voicebank's config would specify the target encoding.
With that version of the plugin up and running, most of the work for the end-user is done, and it's up to them to manually join and split notes. Then attention can be focused on automating that part too!
After that, the plugin could be updated to automatically recognize the format of the UST instead of the user having to do so.
Ganbatte to anyone interested in making this plugin!