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