I have contributed to the document you linked to and have been doing some work in this area, so just a few pointers:
Is there anything happening on this front?
Yes there is.
When will things be up and running for non-ConTeXt-wizards to try out?
Development is driven by demand. It is highly unlikely that some ready-made module for "non-wizards" will be available any time.
Will there eventually be a "how-to" for critical editing in ConTeXt?
IMHO, that's asking for the impossible. There is no such thing as "critical editing," there are hundreds of different traditions in different fields. Again, development is driven by demand - if you can come up with clear and reasonable specs, it is possible that ConTeX can already do some of the things you want and that others can be implemented easily. But given that those demands are almost without limits, no-one will just sit down for the heck of it and try to come up with a universal solution.
Judging from the examples in the pdf it is already possible to some quite impressive editions with ConTeXt
If you're referring to p. 40-50 of the document you linked to: I produced those, and they are handcoded, not produced with any programmatical tools.
I'm slowly starting work on a style sheet that will allow me to typeset TEI xml in ConTeXt, and I hope that this may be useful to other users as well. But again: I'm doing this for my own work, and it is unlikely that this will be a solution for every imaginable critical edition.