I've recently been asked to be a Moderator Pro Tempore on the Gadgets site. I don't know yet if I want to be a full time moderator, or indeed if I'd be wanted as one, but it seems that this is an area of site building just as important as the various other criteria for which badges are awarded on Area 51. So should the pro tem moderators get a badge for their contribution?
A possible reason to not award one is that the position is by invitation only, so basically you're getting a badge at Robert's say so. Perhaps to counter this the badge could be awarded if enough site users say you've done a good job at the end of the beta period. This would mean that to get the badge you would need to be an active and helpful contributor to get offered the position, then judged to be doing a good job by the users once you got it.
Perhaps the "good job" mechanism could be in a format similar to the existing moderator elections where each pro tem moderator is listed and users can vote them up or down.