You want a mandatory lock that the system enforces.

No you don't. At least, not in general. Just suppose you have mandatory file locking. Consider a (user) program that opens /etc/passwd and acquires a read lock. You can't prevent that as /etc/passwd needs to be readable by all.

As long as the program keeps the lock, noone could acquire an exclusive lock on /etc/passwd, so no modifications to the user of the machine are possible.

Believe me, you don't want system-wide mandatory locking.