This might work. From the 'specials' section for #1 Gold on the Hertz website:
"Now through June 30, 2010, you can enjoy a Two Car Class Upgrade when renting a Compact through Full Size car with a minimum 3 day rental at participating locations in Africa, Asia, Australia, the Caribbean, Europe, Latin America, Mexico, Middle East, New Zealand, and Puerto Rico using PC# 140394."
However, I tried a similar PC twice last summer (2009) in Spain and in Germany, and didn't receive any upgrade at all. However, I was renting off-airport, and they probably didn't have much inventory to allow for upgrades.