I guess a proof-of-concept, ad-hoc extension can be written in one day, or even less. But having an industry-ready one is another kind of beast (writing tests, designing the interface, thinking about install and deployment, etc.) [...] For info, see the XSpec test harness I am writing for BaseX in attachement.
Great to hear that you've already spent lots of thoughts on this! In fact we'll need someone who has the endurance to tackle all the usual nuts and bolts, but on the other hand we can benefit a lot from first prototypes. Maybe someone on the list who's working with XProc is interested to have a look at the test harness?
Just as a side note: our XQJ API might as well be one possible interface to connect XProc with BaseX (although we advise most users to apply our own APIs instead..).
Hope that helps, regards,
It sure helps! Christian