I understand "stock" range is 1ms to 2ms with 1.5 being center. But our servos can do 180 degrees if they are driven with 0.5ms to 2.3ms PW. How can I change my RobotC function so it's range maps to a wider range?

Per reference to this thread (viewtopic.php?f=53&t=5030&p=26978), ROBOTC is unfortunately unable to force a pulse wave outside of the standard 1.0ms to 2.0ms range (as the Cortex's Master processor is what actually sends the pulsewaves to the motor ports). The only way to create a wider servo range is to either gear the servo appropriately or to use an additional piece of hardware, such as a 'servo stretcher', to achieve the required pulse width range.

Please be mindful of rule <R5> of the VEX Toss Up Game Manual if you choose to go with additional hardware, as it probably will not be legal for competitive use.