Ah, well that's just raw haxe scripting (subset operators of the high-level programming language Haxe). Wouldn't really be a markup representation of TG functionality, but just exposing TG's scripting. Houdini uses Haxe, and employs Haxe Scripting for hooking. This can be done by just allowing raw programming input in some plane which drives all of TG -- unless Matt really wants to switch languages and rebuild TG.
This is already somewhat available with the SDK, it's just not dynamic within TG.