You could make them a moderator temporarily; I believe there is no pm limit for "staff".
I don't want them to have no limit, I want to set them with with a higer limit. Setting them as moderator also gives them special access to certain forums.
I am also thinking about setting higher limits for users who buy services from our hosting sponsor. Ie, if you buy for them, we reward you with a much higher PM limit.
All in all, lots to say for setting PM limits per user besides the global setting

And talking about PM's. How about allowing an admin/moderator to always send a PM, even if the users box is full. I still want to get my message out.
I now use a trick to temporarly set the PM limit higher before I send it.