Waiting for someone else to comment, I propose this
<assign|rightChoice|<\macro|x>
<math-table-base|<left|.><tformat|<cwith|1|-1|1|-1|cell-swell|<value|table-math-swell>>|<arg|x>><right|}>>
\;
</macro>>
I started from the macro that defines choice (that I got from the "Edit macro" menu command) and I moved both the brace from left to right and the point from right to left. I do not master yet the editing of macros in the TeXmacs source mode, but it worked and it gives what you want as far as I can tell. Let me know if you need step by step instructions to make it work.
G.
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