Damselfish:playing devils advocate here a bit - by definition, people renting tanks have to touch tanks that aren't theirs.
I understand what you're saying here too...
However... any time I've rented Nitrox tanks... I have been required to
1. Write my name on a piece of tape (attached to the tank)
2. Write the O2 percentage of the mix on the label.
3. (sometimes) they also want MOD on the label as well.
So - essentailly - it is my tank for the day.
Also - any time I've rented tanks, they have the HUGE - NITROX stickcers on them. I've (almost never) seen rental tanks w/o Nitrox bands (labels) on them.