Wow, that was a long time ago, very fun project to work on. But yeah, without deep knowledge of the TeX side you'd likely still struggle a bit to understand how the two sides interact, at least it was a bit of a struggle for me, maybe / hopefully it got a bit better in the meantime.

And thankfully LuaLaTex was ready enough at that time, I can't imagine how bad it would've been to do this in pure Tex ...