Can anything replace the 50-500 OS?

You should get the 120-300 F/2.8 OS and carry it around for a few weeks, then when you
go back to the light little 50-500 OS you will think you've got a feather in your hands

A monopod is also a good suggestion, you can get some reasonable priced
carbon fiber ones that will hand the weight of the lens, that colapes quite
small and will fit into your pocket with ease. Take a look at the Gitzo GM2561T
or something similar.

--Evan

Ouch - I just looked at the weight of the 120-300 (2950g)- I never had before and it was on my wish list too..

Well if I even do get one of those I really will have to do those push ups!