The only way i can think of getting reach with out adding weight is via crop factor of a series 1 and FT-1 the 70-300 would crop to a 189-810 mm,but that does have its down side,difficult to get moving subjects,nearly impossible to get BIF,and not all that much scope for cropping in post.
Nikon V2 with Nikon 70-300
As you probably know i use a bridge camera when this old body of mine doesn't want to lug the heavy gear around,you get the reach then but its best used for small birds not too far away and action shots are out.
Nikon P610
The obvious answer is the new Nikon 300pf and 1.4 but at what cost