I have tested mail merge and together with IF fields it works!
At first I was reluctant as I thought I would have to cut the original document into pieces, and put all optional texts in a db field. But when I added IF statements along side the mail merge I could avoid moving larger chapters away from the original.
I am not sure I would recommend this solution to anyone who isn't familiar with code, but otherwise it is pretty simple.
1. Go to tab Mailings and create a Recipient list.
2. Add IF fields that check against the recipient list and remember to add quotes around the text you want to show. (
http://word.mvps.org/faqs/mailmerge/MMergeIfFields.htm)
3. Start Mail Merge
4. Finish & Merge
The one thing to remember is to activate Show field codes (
http://www.extendoffice.com/documents/word/899-word-display-hide-field-codes.html#a2), otherwise it will be hard to control the document when updating it.
A possible downside on this solution is - as you Charles pointed out earlier - that the IF fields can bloat the document if they are many.
Thank you so much for you help!