Wow...I don't think there is a way of thoroughly cleaning the inside of the cowl short of giving it a good blast of water or removing the grill. If it were me, I would remove the grill & detail it by hand. Otherwise, a blast of water and a good air compressor might be the ticket.
I'm not one for promoting jobber's products but I've tried them all (well, almost all) and these guys have good products and fine detailing gadgets that work.
FWIW...here's their detailing tools...http://www.griotsgarage.com/category/tools/detailing+tools.do