Stage configuration (clipping the pony on to front BCD) is a proven method, used by technical divers to carry stage/deco cylinders. However, the effectiveness of that method primarily depends on the BCD used; and whether it has suitable attachment points - many (most?) jacket-style recreational BCDs don't have those points.
However, it isn't "diagonal across the front" - it is parallel to the side, clipped off at the hip and shoulder d-rings (if they exist).
The benefits of stage configuration for pony is that the cylinder is in front of you. The valve and spg can be manipulated and viewed, hose management is neater (hose is stowed along the cylinder unless required), the cylinder can be donned/doffed on demand (passed down from a boat etc).