\advance\parfillskip by 0pt minus \parfillskip
\advance\parfillskip by 0pt minus -1em
</texcode>
Like this:
<texcode>
\installalign
{block}
{\parfillskip=\hsize
\advance\parfillskip by -1.5\parindent
\advance\parfillskip by 0pt minus \parfillskip
\advance\parfillskip by 0pt minus -1em}
% followed by
\setupalign[block]
</texcode>