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: