Ian, The Castle Rock loop is open all year for cross country ski trips. You have to have at least 2 people and check out by radio. You will get a safety lecture once here about that. The back part that drops down actually goes to the McMurdo Ice Shelf which is permanant ice. Once Mid October or so rolls around you will have access to a few sea ice routes. The Armatage trail leaves McM and goes by sea ice around to Scott base. It is an alternate to walking the Scott Base road. Armatage would be sastrugi. If I spelled it correctly. Sort of frozen snow drifts. Once December rolls around the sea ice gets too thin to travel on and they close both routes.
Once the Airfields open, you can ski the ice runway road. That goes straight out from McM to the runway. About a mile or so. Willy field skiway and the road to it are also available.
here is a map of the flagged routes. The ice runway road would go straight out from the bottom of the peninsula