Yes, 16.9 is one of those rules we probably need to tidy up as it doesn't specifically mention if a spare is needed or not.
The way I read it, the only requirements of a spare tyre is that it should be the same size as your other 4 tyres and it be securely fastened somewhere behind the cargo barrier. There's no mention of whether a spare is required or not, or if it has to be on the vehicle while competing in a stage. So ben, I would check with the event organiser (monstr) to see if he has any extra requirements, and if not you could pretty much do whatever you please as long as you comply with those 2 requirements in rule 16.9 ...at least that's the way i see it.
If anyone has any thoughts on 16.9, post up at
http://www.4x4challenges.org.nz/forum, and we'll add it to the list of things to work on for the next version of the rules.
Oh and by the way, the trucks looking primo ben!