Hello Patrick,
This proposal only stops user from entering a wrong combination.
All User-Exits mostly Check such conditions and prevent users from making undesirable entries.
So, it does not overwrite.
In fact, if we attempt for a code that can overwrite, this goes into complex and long BDC code with CALL TRANSACTION IW32.
User-Exits IWO10009 and QQMA0014 are very useful enhancements which are used to check proper entries in Maintenance Orders and Notifications respectively.
Cheers
Jogeswara Rao K