[soLong]

Such big lenses is why SLRs developed into bigger cameras with grips.... Putting a small, not ergonomic "mirrorless" behind it is a bit contrived and backward. 

 

.....not that i might do such a thing, but a small v light quality mirror dslr hanging off the back of it might look like fun....if the somewhat lighter sig(150-600) to come ain't the goods....the small light(quality dslr = no extra weight = so easy to hand hold @900mm