Disclaimer: I don't know javascript or anything about beamng modding. I was poking around in the javascript file for the app but I couldn't find anything that looked like it restricted the minimum speed that could be set. So is it defined somewhere else? If so, where?
Should be lua/vehicle/extensions/cruiseControl.lua - line 8 Beware that changing things in such files could cause unexpected behaviors, make a backup of the file and edit at own risk.