Panton S chair with cracks - how to repair?  

CICS Cologne
I know about a MA Thesis at the CICS - Cologne Institute of Conservation Sciences, Institut für Restaurierungs- und Konservierungswissenschaft
Fakultät für Kulturwissenschaften der Fachhochschule Köln/Cologne that is about this topic of the ASA models of the Panton Chairs. A variety of different methods like tech welding is discussed. The Thesis isn t online yet, but when you will find it here: http://db.re.fh-koeln.de/ICSFH/
http://db.re.fh-koeln.de/ICSFH/

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Carbon fiber flying buttresses . . .
and duct tape.

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I watched a documentary called 'Genius of Design' last week and a Vitra conservator was working on an S chair, it looked like he was performing a cosmetic job though, maybe all thats possible
These links might be of some help, they're interesting anyway, that Institute looks good too but the page won't translate to English.
http://www.axa-art.de/en/artprotect/axa-art-research-grant/plastic-art.html
http://www.design-museum.de/en/collection/restoration.html

Translates for me...
Go to the page linked below and find the "English" link. Don't click on it; instead, drag it to your browser's Bookmarks Toolbar (aka Links Toolbar). It'll make a little button there called "English".
Now, when you want to translate a web page to English, just click the English button (or if you only want to translate a portion of the page, select that portion first and then click the English button).
http://translate.google.com/translate_buttons