For those who are interested, I've finally solved my problem (the vertical space before a subsection which was not automatically swallowed at the top of a page when the page started with an empty section) by using the pair 'after={\vskip 0em\blank[disable]}' for sections:

\setuphead

  [section]

  [placehead=empty,

   page=yes,

   after={\vskip 0em\blank[disable]}]

\setuphead

  [subsection]

  [placehead=yes,

   before={\blank[line]},

   after={\blank[line]}]

\setupheadertexts[section]

\showframe

\starttext

   \completecontent

   \dorecurse{2}

     {\section{My section}

      \dorecurse{2}

        {\subsection{My subsection}

         \input knuth}}

\stoptext