I was be able to book corp rate for Hertz and that is because I have those CDP codes saved in my Hertz online user profile. But I assumed you are referring to Hotel booking, my suggestion is book it with them and then change it online yourself (if possible) or call up the hotel / chain-reservation to change it afterward.
Thats a good idea, just book the BAR with Amex and then when I get there, or by phone call earlier, apply my corporate rate.
Thanks