It's possible to register controls for the PropertyControlClassFactory in web.config. But that works on a property type level, i.e. replacing the property controls for an entire class, in this case PropertyString. Doing it for just one specific property is trickier.
Perhaps you could register a custom control for all PropertyStrings and check if the current property is PageURLSegment. If so, add your special logic, and if not, use the default handling.
Per GunsarfsEPiServer Development Team
Thanks. I did a short test and it seem to work the way you described with PropertyControlClassFactory and special logic for a specific property name.
This is also a way of changing a built in property