There is a freame gussetting kit available from HPN/Wudo and I think Touratech as well. It is only intended to reenforce the main frame. Unfortunatly, this has the effect of stiffening the frame too much and causing it to crack in other places....like in the middle of the top tube. Not good.
The best thing to do with the subframe is to have an identical one made up out of better, higher grade chromoly tubing. High end bicycle tubing would work in a pinch.
Jim Rowley of MAP Engineering
may be able to sell you or at least suggest some alternative/stronger designs.
My suggestion would be to remove the subframe, cut it in the identical spot on the left side, insert a tube/hollow pin between the broken bits, braze it in place or at least cross pin it, and then weld the subframe back together.
What were you doing and how much load was on the seat and rack?
Hope this helps.