Titch, I'm with Pierre... I don't know if the seat tube will be able to withstand the twisting and shear forces of the rear shock. Perhaps moving the mount to a gusset between the seat tube and the top tube, so the top tube takes some of the load?
Of course, your design may be fine. A lot of existing bikes have their shocks mounted to the seat tube, but a one-sided suspension changes the dynamics a bit. Unfortunately, without sophisticated software (or engineering knowledge and a slide rule
) that can analyze the stresses, you're going to have to rely on trial-and-error. And since you may not have the resources to do destructive testing and build multiple prototypes of the frame, you'll probably have to be conservative in your design in order to eliminate the "error" part of the equation. Which means, in the end, the weight-saving benefit of the one-sided design may be negated or even reversed.
But still, go for it. You'll certainly learn something, and you may hit the sweet spot and end up with a great bike.