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.
EPiServer 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