I don't think a righting bag would work well as a sea anchor -- too heavy and not as open at the top. But why would a sea anchor be too heavy to use as a righting bag? You would just let in enough water to right the boat.

Or else you could have a pulley mounted on your righting line that goes over the top of your hull, and run a line from the sea anchor up through the pulley and back down to you to put your weight on. I'm no engineer, but wouldn't that 2-1 purchase increase your righting leverage?

Guess I will have to experiment.