Okay so I gave this an honest try and I've got to say it's a huge headache... I had every aspect implemented and got confirmation that this code was being called: https://docs.unity3d.com/ScriptReference/CanvasRenderer.EnableRectClipping.html
...But it didn't seem to change anything at all about the text being rendered. Unity has 0 docs on how to implement IClippable, and even going into the source code, I'm having trouble telling exactly what's missing. So I'm going to suggest to just use regular masks for the time being, as much as I want this to work. Sorry...