Hello, we just released our VN engine (as it seems that our game which uses it and for which it was created is stalled forever). Just wanted to give you a shout in case you are interested. It's based on custom scripts and has utilities to convert and generate the binaries you include as clusters in your project. Minimal BEX knowledge is advised, and if you are a BEX code the engine is easily expandable via external subprograms created in native BEX which you can launch at any time.
I would have liked to make this in Second BASIC, but it seems that Mega CD support is not planned.
Here's the github repo. I wrote some docs, but I'm not sure if they are enough. Any comments / questions welcome!