I would add to John's suggestions the following observation: First, I'd ease the rigging only enough to get the post up into position. Secondly, I think it would be good to make sure that the wood under the post is epoxied so it will not subsquently rot from above or below. Thirdly, you might be able to get away with digging out the rot and replacing the material with epoxy -- but you might have to cut-it out and replace it completely under the post. I'd seal the base of the wood post too.
Finally, you need the rot cleaned-out, the fungus or mold cleaened-out, and the wood dried. After you use chisel, spoon, wire brush, picks, etc. to dig-out the rot, I'd attack with a hair dryer, then use some rubbing alcohol to help dry it out and clean it, then more hair dryer, and then use a West expoxy and a trowel. As long as the epoxy isn't exposed to sunlight, you may not need to paint it. If you can see it from above, you should put something in ut to keep the UV from degrading it. Paint or some wood trim would work.
Oh -- is the reason that the wood rotted-out was because of moisture from above or because it's collecting in the head bulkhead area from the shower? You should see about clean-up that issue while it's "assholes and elbows" time as you become one with your sole (however it might be spelled).
