Here's pictures of me Rocket Jumping, and me on top of the lightpost thing. A movie of me Rocket Jumping to the top of sidewinder is available at http://www.mythica.org/halo/rocket_jump.mov courtesy of Brian Towne, and the Psyjnir Complex Hotline server, courtesy of Psyrixx and *Ar-Isildur.
First I went to the upper level of the Red Base, and went out on the ledge in the front. On the edge, I pointed my rocket launcher straight down and fired while jumping simultaneously. In the air I held BACK, so I would land on the ledge above the doorway.
MAKE SURE TO LET YOUR SHIELDS REGENERATE EACH TIME!
I did another rocket jump, while holding back and got on top of Sidewinder. From here, you can walk along the rock ridge leading to the post, and fall down behind the red beacon light. Again, I rocket jumped, and in the air I positioned myself to land on the post. Unless someone is zoomed in you look like you're part of the pole!
Obviously high shields (I used 400% or whatever is maximum) is required here. You might be able to use lower, but I haven't messed around with it.