MOST EXCELLENT IDEA, Malte!
In the data model, please add to your list of attributes the following:
1) "category", so we might distinguish between help operating the interface and help solving a variety of educational or content problems: forum use, for example, takes a number of different forms.
2) "level", so we might distinguish between, for example, beginners who want to be intermediate users, intermediates who want to be advanced users, instructors interested in getting at the results, sysadmins/developers who may appreciate advice setting things up and need to know about bugs and idiotsyncresies.
3) "course admin", so each instructor will be encouraged to add comments tailored to his or her group of courses.
As the above list suggests, I think that the ability to easily add and amend the help files to suit different user groups would be a crucial, killer functionality: I want my instructors to be able to contribute to the help files on a continuous basis.
For example, it might be helpful for both the users and the administrators to be able to open up help and find a simple text editor (like Moodle does it, an editor with simple formatting features) where he or she could add a message without leaving the application's context.
I'd love to see a help administration feature whereby I could see all of the help files listed (sorted by alphabetical and most recently edited) in one place so I might develop a systematic overview as well as add/edit to the whole package.
And finally, an export feature.  I am assuming you would be wrapping this up in xml, whereby the Dotlrn community would find it easy to exchange their customized help files and be encouraged to do so.
For me, the key thing about help is that it be context-dependent, multi-levelled, and readily amenable to editing.
I want my users to treat each of the many exquisite moments of pain they often enough experience using our current system as an opportunity to return the favor by offering their ever-friendly sysadmins and developers a little constructive criticism: we might best think of "help" as a two-way street.  For marketing Dotlrn's user-friendly orientation (!), this would be a real plum.  For users, this would be a terrific asset: it would help stitch them into the system, converting a cry of pain into an invitation to connect and improve the application.
May I return the favor, at least in part, by offering to edit any article you might write for "thejournal"?  Thanks for raising this issue!
Bruce