Is the next best option a light rack with gear in a dry bag strapped on top?
Seems like a good idea. There are several different racks out there that are pretty tough, and not too heavy. Old Man Mountain's racks probably the best combination of strength and weight. I've used one, and it was great. Freeload racks are also pretty popular, but I've never used one.
http://www.oldmanmountain.com/http://www.freeload.co.nz/There are a number of seatpost racks out there too. A lot of them seem to break, although I've heard that there are some tougher options as well. Probably the lightest option, but you'll have to do some research to make sure that get one that's strong enough for your load.