David Kean’s comment here gave me the answer I’m using for the moment:
or remove the <ProjectTypeGuid> element entirely – this will opt you of “portable” enhancements, such as a UI for changing the target framework, etc
I’ve tried that, and it works like a dream. On machines which have everything appropriately installed, you can then even build the PCL version under Express! I’ve verified that the resulting binary really is a PCL, and it seems fine.
I wouldn’t be surprised to find that I ran into some problems later on, but for the moment this works fine for me. I can easily live without the enhancements in Visual Studio – it was already confused by my project having very different build configurations, so I don’t think I was getting much benefit anyway.