IIRC, you have to have an ACC (Adaptive/Active Cruise Control) enabled vehicle in order to control the speed over the standard OBD-II port. This is because the vehicle has to have the capability to control the speed already, in order for Cruise to interface with it and adjust the speed. In other words, any car that Cruise can work on, should already have ACC built-in, or at least have optional ACC in that model.
Since Cruise works on any Audi A4/S4, I'm guessing Audi uses the same base control component for all 4 series, so that even for a vehicle that does not have ACC or Auto Parking enabled, Cruise can still interface with the vehicle and adjust steering wheel and speed. This suggests that upgrading your car to support ACC might be feasible?
Elsewhere in the thread, Cruise's co-founder indicates that they're using the old-school mechanical automation method: actuators on the brake, wheel, and throttle.
Most cars with active cruise control don't comply with any standard - while the commands to accelerate, decelerate, or steer may be available over a CANbus which is exposed on the OBD-II port, there's no standard like there is for OBD-II diagnostic information and the protocols are often obfuscated or encrypted (either for "security" or manufacturer protectionism).
Since Cruise works on any Audi A4/S4, I'm guessing Audi uses the same base control component for all 4 series, so that even for a vehicle that does not have ACC or Auto Parking enabled, Cruise can still interface with the vehicle and adjust steering wheel and speed. This suggests that upgrading your car to support ACC might be feasible?