[NTG-context] offset for background overlay

[mf]

Hello list,
this MWE is a simplified version of a real case:

\startuseMPgraphic{cell:triangle}
   path p ; p := unittriangle rotated 90 xscaled 2.5 OverlayWidth 
yscaled 2.5 OverlayHeight ;
   draw p withcolor red ;
\stopuseMPgraphic
\defineoverlay[triangle][{\uniqueMPgraphic{cell:triangle}}]
\starttext
\bTABLE[frame=off]
   \bTR
     \bTD[nx=2,background=triangle,align=middle] \dontleavehmode a\crlf 
text \eTD
   \eTR
   \bTR[toffset=1mm]
     \bTD left \eTD
     \bTD right \eTD
   \eTR
\eTABLE
\stoptext

The triangle is used as background of the 2-column wide cell of the 
first row.

I want the triangle to go around the second row too, as if it were a 
frame for all the 3 cells. It means that it should be shifted down a bit.

So I tried this:

\startuseMPgraphic{cell:triangle}
   path p ; p := unittriangle rotated 90 xscaled 2.5 OverlayWidth 
yscaled 2.5 OverlayHeight ;
   draw p shifted (0,-3mm) withcolor red ;
\stopuseMPgraphic
...

but it does not work, because the content of the background is centered 
on the cell, so "shifted (0,-3mm)" has no effect.

The only solution I found is drawing something above the triangle with a 
neutral color, so that the whole background is centered in the cell and 
the red triangle goes down:

\startuseMPgraphic{cell:triangle}
   path p ; p := unittriangle rotated 90 xscaled 2.5 OverlayWidth 
yscaled 2.5 OverlayHeight ;
   fill p shifted (0,6mm) withcolor white ; % only to move the next one down
   draw p withcolor red ;
\stopuseMPgraphic
\defineoverlay[triangle][{\uniqueMPgraphic{cell:triangle}}]
\starttext
\bTABLE[frame=off]
   \bTR
     \bTD[nx=2,background=triangle,align=middle] \dontleavehmode a\crlf 
text \eTD
   \eTR
   \bTR[toffset=1mm]
     \bTD left \eTD
     \bTD right \eTD
   \eTR
\eTABLE
\stoptext

It's a dirty trick. I'm sure there's a cleaner way.
Does anybody know it?

Thanks,
Massi