Whenever I go on a driving ski trip with the whole crew (girlfriend and my 2 kids and her 2 kids) we rent a minivan and I have never hesitated to put chains (actually, z-cables) on the van as and when necessary. To the extent that this violates the rental agreement (I've never actually read all of the fine print in the rental agreement--I'm sure that there are a lot of things that it prohibits that everyone does) I'm willing to risk it on the grounds that:
1. I'd rather be mobile and safe than be immobilized but in compliance with an arbitrary and unreasonable restriction (if the chains/z-cables come off and damage the car, the rental company will make me pay for the damage, right? so what's the rationale for not letting me put the chains/z-cables on?).
2. How will the rental company ever know?
3. In situations in which the rental is at a "Ski Airport" (Reno, Salt Lake City, Denver, etc.) there is somewhat of an implied consent by the rental company to the use of the car for skiing, and this commonly will include the use of chains/cables.
I would be interested to know if any rental car company has ever actually made a claim against a renter for putting chains on. I don't think that would be very good PR.