XProc 3.0 User-Defined Step

Hello !

first trials to XProc 3.0 start !

In our pipelines, we need to request MarkLogic via XQuery. At this time, we use a Saxon XPath extension function from an XSL.
I'm looking for the best way to do this with XProc. 
I could continue to use my XPath extension function, as both Morgana and Calabash support Saxon. But, is it possible to provide a user-defined Saxon configuration to both implementations ?I could write a user-defined step in XProc, using a Rest API of MarkLogic, but it is not as easy to use as the direct ML client, and does not allow all requests.I could write my own implementation of a MarkLogic step, but I have no idea of where are negociations around a common API for implementing user-defined steps.I could expect implementers to suggest me another method.

Which way would implementers incite me to choose ?

Best,
Christophe