Not possible as I see it.
There are one or two manual ways,
1) - when the limit is reached, simply lock out the form. You can put a custom "form is closed" message on it. (not ideal, you said the client doesn't want that).
2) was thinking you could do it with conditional logic (as in "if [number of entries = 200] then Display [this thanks page]"
Otherwise, you could just monitor the number of entries, then on reaching the limit, adjust the "thanks for your submission" (custom message) page to add the notice about being on the waiting list.