The following example (mkiv only!) by Wolfgang Schuster shows a table using MetaPost for the rules:
<texcodecontext source=yes mode=mkiv text="gives:">
\startuseMPgraphic{tableborders}
\stoptext
</texcodecontext>
See the corresponding thread on the list: [http://www.ntg.nl/pipermail/ntg-context/2011/058549.html rules with different thickness]
\eTR
\bTR
\bTD Text 3 \eTDpasteeTD
\bTD Text 4 \eTD
\eTR