The Beto Rack looks the most promising for now. I ordered one of those. I figure if it's rates to support a kid and a child seat, it'll certainly hold a light touring load in a trunk bag. Thanks, havm66z.

If that doesn't work out, I'll try the seatpost racks.

enargyandair, the problem is that the bag will eventually be dragging on the rear wheel. Extending the rack an inch or so might do the trick, though.