Hello Rowan!
I've just read about the push for wpa81 support in nuget on the .NET Blog, so it's already on my mind. Thing is, I don't have the RC installed, so I can't test the APIs. Also, just changing the nuget package won't affect the assembly-level TargetFramework attributes, which means I would actually need to have the Update 2 RC installed on my production system and that's not something I feel comfortable with.
So, here's what I can offer: I'll set up a test-system with the Update 2 installed and if it's enough to just patch the Reference Assemblies folder on my production system and/or to manipulate the nuget folder, I'll do that. Otherwise, Windows Phone 8.1 will need to wait till RTM. Either way, this is going to take till into next week.
Best regards, Michael