I don't see where the PHP is needed in that kind of setup.
At the core, what's the difference between the help page and, for example, a forum's post? As far as I can see, only permission and the ability to use specific bbcodes and/or html in it (for headers, code example, anchors, etc.).
The basic admin would still have nothing to do, the appropriate localization would be loaded (the "post" would be populated with it) when the forum is installed and that would be it. The average/a little more advanced admin would simply have to edit the "post"n as any other post, only with more freedom. And the advanced admin could do his own trick.
The only kind of code I think would be needed is for the smiley list, to generate automatically the correct and up-to-date array of images|meaning|code. But that could be done with a simple string replacement, something like [smiley_list] that would be replace with the smiley list.
Adding a simple hook at the end still require of the admin to know PHP, and wouldn't be enough for anything but "stand alone" extension. If for example you add a new bbcode tag, you need to add something in the bbcode section of the help. If you change a bbcode working, or even delete it, you need to edit that section. And so on.
And I guess it will need some kind of [nop][/nop] special bbcode, with the enclosed string not parse by bbcode and escaped to avoid html; and maybe some kind of header string to divide sections and place anchors.
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