I haven't done this repair myself, but I did take my '05 4.4i apart this summer to replace the valve seals, hoses, water pump, water pump transfer pipe, etc, and looked into replacing this seal as an additional preventative measure. I wanted to replace every rubber gasket I could while I was in there. I still have the rubber seal ready to go.
Since it wasn't leaking on mine, I decided to leave it alone as it will be a major PITA to get too.
It is apparent once you get access to the alternator mounting plate that you have to hold the engine to remove the engine mount on the drivers side, and you will have to remove the front differential. The front differential is mounted to the engine block right in front of the bolt that need to be removed to remove the alternator mounting bracket.
Once the front differential and engine mount are out of the way, it shouldn't be too difficult to change the gasket
There is a write up on the procedure somewhere online. I can't remember where I found it. Probably on here, or on Bimmerfest.
Good luck.