Here is a work-around that Charles Culp discovered and showed at SWW 2012. Alin Vargatu has a video illustrating it, but I wasn't smart enough to bookmark it. If I understand it correctly, you can put a sketch in your offset plane and add three points at the origin, at X=0 and some value of the Y and at Y=0 and some value of X. Then define a new plane by picking the three points. Pick them in the clockwise direction. If the new plane is oriented the same as your Top plane you are OK. If it is backwards, then pick them in the counter-clockwise direction.
You could skip making the offset plane if you use 3D sketch points.