thats a pretty good point actually unless you are already carrying 2 cameras of courseI would expect it to be in the region of the 200 f/2 which is handholdableYou know it's a ridiculous idea for a lens when you have to state that yes, indeed, it would be hand holdable when on the short end it's standard lens and on the long end it's a short telephoto. The 200/f2 is like five and a half pounds! A 5.5 pound portrait lens you're whipping around the studio!
It would cost less and weigh less if you just bought a whole 'nother camera and carried both. Just get an extra 5D2 with a 135/f2 to go along with the 50 prime you probably already have.