Rivendell has a very good one made by Nitto in Japan, called the Mark's Rack:

http://www.rivbike.com/products/show...k-nitto/20-108

You can use it together with a low rider such as the Tubus Nova. I have this combination on my Rocky Mountain Sherpa at the moment. While the Mark's Rack is not made for heavy weight, I'm sure it could handle a lightweight backpacking tent of the sort most people would use bicycle touring with no problem. You need the front hole at the bottom of the head tube to mount this one.

Neil