I don't think I'll have time this week to submit a patch, but feel free to ask me later if you can't find anyone with time to do it when you're in the sprint to release Ogre 1.10, I'll be glad to help.
Also, someone should fix the indentation... It seems currently it's an anarchistic mix between tabs and spaces, which means that anyone with tabs other than 4 get a horrible, horrible mess trying to read the Ogre source...
As far as I can tell, no harm should come from this today. Granted, the dev team wants Ogre to build on every platform, but sometimes true standard-conforming code isn't the better. Include guards are, from my opinion, a good example of this.
Unfortunately the lack of guarantee from the standard makes it a potential risk when porting for new plateforms with compilers that don't have (unprobable) pragma once or implement it in a different way than most compilers (more probable).
So even if for the sake of simplicity frankly we should do it, without guarantees from the standard, it opens a hole in the portability issues wall.