Will I have access to any lounge at Keflavik airport?
The SAS Plus page you linked, does mention the access to lounges. However it does also note that this access is subject to availability.
The only lounge at Keflavik seems to be the Saga Lounge by Iceland Air. In its access rules, Iceland Air does not mention SAS passengers at all, regardless of flight class. Hence, SAS Plus passengers do not have access to the Saga Lounge at Keflavik. For more information on the SAS partner lounges, see the dedicated webpage by SAS.