I plan to do this with my rollcage in the back.
The seatbelt bolts are not metric like the rest of the car. 7/16 UNF Thread. There would be an adr standard somewhere for them, and i think it would depend on the engineer how he wants you to mount them. Belts also need to be adr approved.
I would also put some baby seat points in the back also while you at it. Not that a baby is safe in a zook anyway, only for emergency
Hopefully this page will help you in the right direction
http://www.tmr.qld.gov.au/Safety/Vehicl ... tions.aspx