I am looking for a way to create paragraph numbers (in German: Randziffern;
dict.leo.org calls them "recitals") that are written in the page margin area for
every text body paragraph of a book. 

The numbers should be on the left of the text on left pages and on the right of
the text on right pages, and there should only be one number on the top line of
every paragraph. I not talking about line numbers!

Up to now I have used frames anchored to the first line of each paragraph
containing a text in a numbered paragraph format as a workaround. But you have
to insert such a frame to every paragraph by hand, it is a hassle to create
cross-references, and the frames are very fragile (i.e., if you by accident
select a frame and press a cursor button, the frame shifts to a different line.
So the risk of printing errors is high.) See the attached example for my current
status and the url for a discussion to the topic.

This way of numbering paragraphs is very common in legal texts (mainly in
German, but also in other European legal texts). 

It should be possible to achieve automatic margin numbers by extending the
paragraph numbering feature by a "mirror paragraph number on odd [even] pages"
option like the one that exists for frames (see the cited discussion for details).

Furthermore it is impossible to create an automatic index which references the
recitals instead of pages. See the corresponding issue I'm just going to add to
the database.