You are thinking this backwards. 2NT with an unpassed partner has all the upside good hands(near/full openers =slammy) to counter the busts. With a passed partner you are mainly keeping the downside cases. Use 2NT in 3-,4-seat for special cases. I use 2S, 2NT as namyats: solid/near solid with a void. Alert partner 1xA may be slammy. Then namyats 4m has no void -- A-ask worthwhile.