There barriers aren't really necessary. Sure there is the possibility that not everyone will be online, but as mentioned in another topic, some villages (Including Otogakure) require you to send a message saying you're attacking. From there the kage and attacker can organize the rp event. It should usually be several days before the attack so the village can tell their members to be on at that time. The village however, still has to act like they did not know of the attack and can't use unfair meta gaming.
Example:
Bocc wants to attack Otogakure. He messages me and we plan it out and make a set date and time. I tell my members to be online (prior notice) This allows more defenders to be online for the attack and not four random chuunin. However, I may not do any unfair meta gaming like telling my members to all learn fuinjutsu to seal his zombies up, or anything else of the sort.
If this system is utilized, there is little need to have constant op barriers.