Hello,
Hm, I'll look into this but I did recently add basic mask compatibility to all the shaders that come with STM, so basic masking should at least work.
Looks like for RectMask2D support I just need to implement the IClippable interface (https://docs.unity3d.com/2018.3/Documentation/ScriptReference/UI.IClippable.html) in addition to IMaskable on Super Text Mesh in order to make it work... I think it should be possible judging by the documentation, but I won't be able to look into this until at least Monday.