Inanek, the frame is a mid-80's mountain frame. Everything was overbuilt at that time. Stays are thick and I am unconcerned with failure of the tube. Using as a drag brake there is no harsh stopping forces being applied all at once, rather it is a steady force caused by dragging the pads on the rotor to control speed. Very different kind of stress being applied to the seat stay than a full on stopping brake.
Anyway, it will be interesting to use in the mountains in the coming years. Unfortunately this coming summer will not test it fully, but the following summer, if all goes as planned, the brake will be tested thoroughly.