Hm I suppose that would be "superTextMesh.leftoverText.Length > 0" as an equivalent to that boolean, but also if you set the cut off mode to "Auto Pause" or "Auto Pause Full", Super Text Mesh has that built-in if I'm understanding what you want to do correctly. Just call superTextMesh.Continue() to have it keep reading past the pause. (This works with the <pause> tag, too!)
What do you mean by "built in test printing functionality" and the scripting system in your text? Are you using your own tagging system? STM should be compatible with a lot of stuff since it's programmed to be generic in use, so would it be possible to use that? You can use pre-parsing in STM to convert your own tags to things STM would recognize if you have an entire script written up for your game already! I may be able to point you in the right direction if that's the case!