You might be able to pull off a PCL using bait'n'switch, but you won't be able to access/expose any functionality of the underlying runtime, but need to wrap everything in PCL compatible types.
However just note that bait'n'switch doesn't give you one binary for 'n' platforms, but gives you 'n+1' binaries, as opposed to a shared project giving you 'n' binaries (albeit you can then package them into a single nuget reference which would be the only way to really use bait'n'switch).