What I've done is to play with mathalignment and \framed. Here is a MWE:

\starttext
\defineoverlay[midrule][{\blackrule[height=.02cm,width=5.5cm]}]

\framed[frame=off,background=midrule,boffset=.65cm]{\startformula
  \startmathalignment[n=12,align={middle,middle,middle,middle,middle,middle,middle,middle,middle,middle,middle,middle}]
    \NC \NC S_6 \NC= \NC 1 \NC+ \NC 2 \NC+ \NC\dots \NC+ \NC 5 \NC+ \NC 6 \NR
    \NC + \NC S_6 \NC= \NC 6 \NC+ \NC 5 \NC+ \NC\dots \NC+ \NC 2\NC+ \NC 1 \NR
    \NC\NC 2S_6 \NC= \NC 7 \NC+ \NC 7 \NC+ \NC\dots \NC+ \NC 7 \NC+ \NC 7 \NR
  \stopmathalignment
\stopformula}
\stoptext

Is there a simpler way to typeset something like this?

Thanks in advance!