Hi,
I believe that the lock function of the ENET locks based on the unit descriptor. Although the lock will not work for thread synchronization, I think this is assuming you are using the same unit descriptor. I haven't tried using separate unit descriptors within the same process.
In any case, the iblck function is implemented only with the ENET/100. For the GPIB-ENET use the iblock and ibunlock functions.
Refer to the following KB:
Locking and Unlocking a GPIB-ENETDiegoF
National Instruments.