> The example you have provided works well for section titles, table of contents and references. What about bookmarks? The following extension of your example produces a bookmark "1 chaptertitle{Long title for the chapter}{Short title}":
> 
> \setupinteraction[state=start]
> \placebookmarks[chapter]
> \defineselector[chaptertitle][max=2,n=2]
> \setuphead[chapter][textcommand={\setupselector[chaptertitle][n=1]}]
> \starttext
> \startchapter[reference=sec:short,title=\select{chaptertitle}{Long title for the chapter}{Short title}]
> \input knuth
> \about[sec:short]
> \stopchapter
> \stoptext
> 
> For setuphead, is there something like a "textcommand" key for bookmarks?

You can set the text for the bookmarks with \startchapter[bookmark=…,…]

Wolfgang