For CubeCart 3, I have experience creating an exporter that creates a QB IIF file. It worked well enough. (Note: the IIF file has its own format and requires an enormous amount of in-line processing.)
I think the CC6 version of exporter methodology is better suited to create the QbXML file.
Recent conversations dealt with a custom module that solved specific requirements. That's why there isn't a "complete" exposition on how to do this.
Also, there aren't any development documents to be had. However, the /modules/external/sage module is a decent place to explore.
We can take this as far as you need by email.