% (the following brace is a modified version of one posted by Wolfgang)
\startuseMPgraphic{accolade}
z1  = (-233,0) ;
z2  = (-220,22) ;
z3  = (-155,211) ;
z4  = (-200,690) ;
z5  = (-142,1033) ;
z6  = (-97,1106) ;
z7  = (-71,1125) ;
z8  = (-61,1104) ;
z9  = (-80,1077) ;
z10 = (-108,1002) ;
z11 = (-129,666) ;
z12 = (-117,207) ;
z13 = (-197,19) ;
z14 = (-208,0) ;
z15 = (-197,-19) ;
z16 = (-117,-207) ;
z17 = (-129,-666) ;
z18 = (-108,-1026) ;
z19 = (-80,-1077) ;
z20 = (-61,-1104) ;
z21 = (-71,-1125) ;
z22 = (-97,-1106) ;
z23 = (-143,-1031) ;
z24 = (-199,-673) ;
z25 = (-155,-211) ;
z26 = (-220,-22) ;
fill z1 for i=2 upto 14: .. z[i] endfor & z14 for j=15 upto 26: .. z[j]
endfor .. z1 & cycle ;
\stopuseMPgraphic

% braces are scaled to fit the box
\defineoverlay[leftbrace] [\middlealigned{\scale[height=\overlayheight,width=\dimexpr 3pt+.015\dimexpr\overlayheight\relax\relax]{\useMPgraphic{accolade}}}]
\defineoverlay[rightbrace][\middlealigned{\mirror{\scale[height=\overlayheight,width=\dimexpr 3pt+.015\dimexpr\overlayheight\relax]{\useMPgraphic{accolade}}}}]

\starttext

  Context version: \contextversion.

  \blank

  \bTABLE[framecolor=orange]
    \bTR
      \bTD a \eTD
      \bTD[nr=3,width=1em,background=rightbrace] \eTD
      \bTD[nr=3,align=lohi] abc \eTD
    \eTR
    \bTR
      \bTD b \eTD
    \eTR
    \bTR
      \bTD c \eTD
    \eTR
  \eTABLE

  \blank

  Following text: \input sapolsky

\stoptext
