It's fair to say that the majors (which are pretty much the only ones renting at airport locations) are optimized for the 95% use case. They work fine most of the time but, in my experience, are pretty much incapable of dealing with anything outside of the usual "system."

In addition to not being able to guarantee a specific vehicle--even with a deposit--I had an issue with Avis a few years back where they were utterly unable to extend a rental to bridge it to another reservation I had already made, i.e. they couldn't take two one week reservations with a week in between and turn it into a 3-week reservation.