\font\rm=cmr10 at 20pt \rm
\baselineskip=5pt \lineskiplimit=-\maxdimen
\parindent=0pt

\setuphead[section]
[style={\rm},command=\bum]
\def\bum#1#2{%
\vbox{%
\baselineskip=5pt
%\rightskip=0pt plus 1fill
#2
}}

\starttext
bum\hfill\break
bac

\bigskip\bigskip\bigskip

\vbox{%
\rightskip=0pt plus 1fill
bum\hfill\break
bac
}

\bigskip\bigskip\bigskip

%\tracingall
\section{bum\hfill\break bac}
\stoptext
