I don't know the answer to your question, but I do know you can pay for access to the HUB lounge at ICN. I recall the cost was something like 30,000 KRW but there were ways of getting a discount I believe if you were flying OZ or KE. Not 100% sure about the discount nor the exact price, nor if that's all day, or just for a limited number of hours. There's no showers in the HUB lounge, but are showers outside of it which are "free" except you have to pay for your towel/shower kit.