i wouldnt put too much time into .lock system. it can be easily bypassed if you put 5 min of thought into it. making it work would require changes to core game mechanics which should be out of question/ability for you.
example. locked char can attack flagged characters. but l2 evaluates non-force attack before start of the attack and pk status at the moment your skill lands. so if i stack -cast speed, grade penalty, and -cast speed debuffs i can easily start casting while target is purple and land hit when target is already white. resulting in karma.
for sure .lock is interesting feature and might stop few scammers/thiefs, but wont rly stop someone with dedication.