The term "grand slam capture" may have come from David Chamberlin's 1984 book "How to Play Warri" which is on line*. Interestingly in Chapter 1** he gives different rules from any others that I have read:
David Chamberlin wrote:
If one player empties the other's houses through a capture, he must move again to provide the other player with seeds, if that is possible. This situation usually arises in a "Grand Slam" capture, in which a player seeds his opponent's entire row twice.
This book is listed on http://www.oware.org/ (but is out of stock).
The Oware Society suggests that a move that captures all the opponents seeds does not capture anything.
So long as there are no houses with less than two or more than three seeds, in-between them, a maximum of five houses can be captured in this way. Beyond five houses one forfeits everything, as this would leave the opponent with no seeds to play with.
Whilst this could be interpreted to mean that you could capture 5 houses this way if the opponent had no seeds in their 6th house I suspect the intention is that wouldn't be allowed (if it were ever to occur in a game which it might not between competent players).
(I can't work out how to make these links work)