If the liverock was in a tank without fish for a couple of days, the infective trophonts of ich would not have been able to find a host and would die. The bigger risk would be that the rock may not be fully cured and could cause a spike in ammonia. If the LR was sitting in the LFS tank for a couple of weeks, I would add it to the reef tank.
.....and hope it does not have killer polychaetes, aitapsias or mantis shrimp.
Mike