Hey!
Through the use of automatically-generated custom events, STM does support furigana, or more generically, ruby text!
Super Text Mesh comes with a simple ruby text helper script included in the examples folder (although maybe I should move this to "Utilities" in the next update...) and I've uploaded the code, too: https://pastebin.com/XGKdPccN
Here's a video of it in action:
I just started learning Japanese myself, so I now know... because of all the varied pronunciations behind the 2000+ regular-use kanji, to type up a word like... "楽しい", you'd have to type... "<e=rt,たの>楽しい "! I bet an automatic dictionary system would be possible someday, but my lessons aren't exactly that far enough along yet...