What about a why bother category?
I just don't see rally as being able to even utilize such a promotional tool right now.
There is a lot that can be done that won't be nearly as controversial and probably cost less too.
A score of umbrella girls will likely cost over $5,000.00 per event, or more, and no, using interested college girls wouldn't be a real option. If you're going to have young women prancing around in seductive clothing you want someone that can really sell your product, not be a distraction...
Using that $5,000.00 for expanding print media (how about guided media tours for local media to provide more insightful and hands on looks into stage rallying to build a better understanding) coverage, paying more to the PR firms for more aggressive solicitation of print coverage, or how about pumping 5k more into the TV budget per event. That would be more camera men or perhaps a helicopter (most helicopter companies charge 300-500 per hour, 5k could easily cover a day of helicopter coverage, and I don't think anyone would argue against good aerial coverage of a rally car being one of the most exciting and specactular things in motorsport).
Umbrella girls have a place in motorsports marketing but North American rallying isn't really in a position to exploit the benefits. This assuming that umbrella girls have soemthing tangible to offer stage rallying to begin with, which could be argued convincingly from either view point without one emerging a clear answer.