Unfortunately there is no way to change the frequency the PWM works at. The only thing you can do is change the Duty cycle, which is what the PWM command does. The only way to lower the PWM freq is to use an external counter or divider. There is one at BaneBots.com that you can purchase that will divide a signal down by 2,4,8, or 16.
Are you saying that even a command of PWM 1:1, or 1:5 make the ESC run at full speed?
Ringo
The problem with changing the freq is that it might affect other systems. The PID, Servo, sonar, etc all use internal timers and I will have to dig to see what is linked to what. Let me see if it will go that slow, there is a limit as to how slow it can go, but I'm not sure what it is right now.