Thank you for the tip, @pireddag!
I solved my problem by following your instructions and writing the following to my my-init-texmacs.scm
:
(delayed (lazy-keyboard-force)
(kbd-map (:mode in-math?)
("math t" (make 'tabular*)
(table-set-valign "T")
(with old-mode (get-cell-mode)
(set-cell-mode "table")
(cell-set-halign "L")
(set-cell-mode old-mode)))
))
Your solution from the post unfortunately had some shortcomings: the cursor was positioned wrong and if an expression was highlighted it got deleted; with the code above, the highlighted math gets put inside the cell of the table.