Both Slann and Stewart are correct. I won't quote the rule directly because I don't have my rulebook in front of me, but I do remember participating in a discussion about this rule a while back. If my memory serves me correctly, the book says that because of the palanquin that the Slann may have LOS as if he were a large target. I don't remember any turn phases being listed, where this doesn't apply and it would seem that the benefit would be indefinite (or as long as the Slann was on his palanquin) as it is a result of him riding on his palanquin.
The interesting question here is that if the Slann is in combat can he still use the staff during the shooting phase? I will take a look at my books after work if I have time and see if there is anything mentioned in the rules that helps clear this up. Something that would definitely sway the answer either way is if there is any rules defining wether characters in a unit that is in combat (but not in the front rank) are able to use items or fight at all. It seems that characters not in the front rank, can not fight or perform actions. The Slann is given express permission in the Army Book to cast magic while in the 2nd rank of a TG unit, but it says nothing about shooting.
If someone has access to their books and wants to shed a little light on the discussion instead of speculating like me, I'd be interested in getting a opinion directly from the book.