These sorts of cars were definitely on a 'do not upgrade list' that was published a couple of years ago, but don't know current rules in this regard. I remember the Volvo and Hummer being on that list, but as observed above that loosened up a great deal with the Hummer when people stopped wanting those due to expensive gas.

I don't usually rent at stations that have anything nicer than Cadillacs, the one time I walked past a Corvette Convertible I joked about being upgraded into it and the CSR smiled back as she walked me to my already assigned vehicle..