The person is withholding two cars when, without a doubt, the intention is to only rent one. And the only reason they are withholding two cars is in hopes of scoring a free upgrade on the first (cheaper) rental before needing to fork out more $$$ for a car they actually need.

Should I make phony fully refundable reservations for all F seats on my flights so I can have a better chance of being upgraded when I cancel the phony reservations at boarding time?

What's your definition of sleazy?