I'm using a local, non-airport, location, so the choices are probably more limited.
I can't seem to find a way to add notes to my reservation. Is there a successful, proven way to request a specific car?
You cannot request a specific car, which really sucks. The best way would be to call the branch the day of a rental and see what they have available.
I rent in the East Bay very often btw and the car selection varies a lot.
Additional tip - Enterprise local branch cars are dumped their from the airport. They have higher mileage and may have mech issues (like alignment). If your car does have a mechanical problem, you can swap it out at SFO or SJC pretty easily. SJC has one of the better car selections last time I was there a few weeks ago but it really depends on time of day and day of week as well.
PCAR - I saw a almost new black 300C at SJC last time I was there. They also had a G37.