Hi Ravi,
thanks for sharing your experiences.
I've now decided to go for a change framework implementation to block all actions in MRS if an order is in status LCK.
One addition to the list of my initial post - I found a third option:
3) Custom implementation of BAdI /MRSS/RAC_SEARCH_ALG. This BAdI controls the access to and partly the interpretation of customizing values. One could copy the default implementation class and change the method ACCESS_STAT_CTR to introduce custom wildcard values.
I don't like copying SAP objects as required for option 3)...therefore I end up with the change framework.
Cheers,
Manfred