In the past I have used .NET with bookmarks in Word.docs and form fields on PDFs but, they both have speed issues IMO.
I would like to use some combination of XML, HTML, and/or XHTML but not finding much in the way of a useable framework.
Probably a side note (rant) and not an answer to your question, but Word should provide an out of the box solution to this kind of problems. With Open XML this should be very easy to implement anyway, but unfortunately the Quick Part button on the ribbon is anemic to say the least (the last time I created a letter template with a repeating Content Control, I had to install the Word Content Control Toolkit and do some mapping!).
I hope the Word team read this and spends some time adding this kind of functionality instead of adding more picture effects...