Depends on the car. Some of them use vacuum to maintain your speed, putting it out of reach of influence of your car PC. The others that DO use the ECM won't be all that willing to allow you to input your own values to set a speed, unless the AutoTap or other software gives you that ability. The thing is, those programs provide an interface command structure to the actual programming language of the ECM, and last time I checked, none of the auto companies take kindly to a private entity trying to break their proprietary property.
In the end, you can monitor your speed, but it may be next to impossible to set one of your own with currently available (private sector) technology.