Just an addition to my above post. I was looking through the NZCAA website for some thing entirely different but came upon something that may be of interest. CAA's around the world seem to quite often have many similarities in their legislation due to I guess ICAO standards. So, this may well be similar to legislation in Europe.
There is an Advisory Circular AC21.05:
Approval of modifications covering aircraft ferry fuel systems and overweight operation.
Essentially, it states that the MTOW published in the AFM can be increased by up to 10% without requiring any engineering modification. The aircraft must of course remain within the published weight and balance limits. I suspect this would require the placement of the ferry tank where the middle row of seats normal live.
website is....
http://www.caa.govt.nz/
go to Rules and more, then Advisory Circulars and there it is.