In natural tables
http://wiki.contextgarden.net/TABLE

the parameters
columndistance=dimension,
and
spaceinbetween=dimension,
will set the gaps between columns and rows, respectively.

Is there some way of specifying row gaps for specific rows?
For example, say that I only want a gap after the first row.
I tried:

\bTABLE
\bTR [spaceinbetween=.25em]
\bTD 1 \eTD
\bTD 2 \eTD
\eTR
\bTR
\bTD A \eTD
\bTD B \eTD
\eTR
\bTR
\bTD a \eTD
\bTD b \eTD
\eTR
\eTABLE

Alternately, I tried
\setupTABLE [r][1][spaceinbetween=.25em]

but these do not work.

One might think about trying

\bTR [bottomframe=on,framecolor=white,rulethickness=.25em]
...
\eTR

but this presents several problems:
1) When using background=color, the background color gives a hairline
overflow beyond the rule, at least using the pdf viewers that I have on
my computer.
2) there appears to be some interference between setting rulethickness
and columndistance, so that spaces between columns disappear when
putting a thick bottomframe. Of course, one could kludge a column
separation using a rightframe, but this is not satisfactory.
3) The use of a white rule when one really wants to leave some space is
not very satisfying.

Does anyone have any suggestions?