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I've ended up doing it in two discrete steps:

1. open all buffers from the current (tree) search (may take a few seconds):

(defun notmuch-tree-show-all-messages ()
        (interactive)
        (goto-char (point-min))
            (while (and (not (eobp)) (not (get-buffer (notmuch-tree-get-message-id))))
              (notmuch-tree-show-message-out)
              (previous-buffer)
              (forward-line 1)))

2. multi-occur only in the opened notmuch messages:

...