I like your way of thinking however I am going to play the devil's advocate here.
The cert plate will then state the size wheel that you have on the car at time of certification. I am guessing the borrowed wheels will be a 16x7 or 16x7.5.
Technically speaking if the car is cert'ed for that size wheel then you put the 16x9's back on, you can get fined, green stickered or whatever they do for having wheels that do not comply with the cert plate.
I only know all this from past experiences