Obmalloc now has theoretical support for locking.  I say theoretical because I'm not convinced it's ever been used.

The interface is defined through five macros:
    SIMPLELOCK_DECL
    SIMPLELOCK_INIT
    SIMPLELOCK_FINI
    SIMPLELOCK_LOCK
    SIMPLELOCK_UNLOCK

Internally these are used to define an actual lock to be used in the module.  The lock, "_malloc_lock", is declared, then four defines are made building on top of the SIMPLELOCK macros, named:
    LOCK
    UNLOCK
    LOCK_INIT
    LOCK_FINI

LOCK_INIT and LOCK_FINI are never called.  So unless your lock doesn't happen to require initialization or shutdown, this API is misimplemented.

Victor: this was your work, right?  If not, sorry, please unassign/de-nosy yourself.