On 2013–03–21 H. van der Meer wrote:

> I would like to put something directly after each chapter number, like so:
> 
> Chapter 5 <add something everytime> Title of Chapter
> 
> Is there an option for this? \setuphead? \setupheader? which parameter?

http://article.gmane.org/gmane.comp.tex.context/77262

There should be more examples on the mailing list, search for
\defineheadalternative

Marco