Yes you can. Just after getting off the bus at muscat airport don't go to immigration counters. just follow the sign saying flight transit and proceed to connecting flight counter after taking the stairs and then present them with your incoming flight boarding pass(it is very essential to show it to the counter staff) and your ticket for the return to AUH. They will provide you with the return boarding pass on the spot and then you will go through security and then you may proceed to the lounge.

By the way, the information provided above are based on actual experience.