Mark,
I like the idea of the workflow you are suggesting. Asciidoctor is a tool, and like all tools, it can be combined with other tools to get the job done most efficiently.
I'd be happy to see a Scribus converter in Asciidoctor. I encourage anyone interested in creating it to use the Mallard converter as a reference, which also converts to an XML format. See
https://github.com/bk2204/asciidoctor-mallard
Cheers,