I have added a WhitelistPlatforms entry to whitelist “UWP32” and “UWP64” in my plugin’s .uplugin file and rebuilt the module, but I’m still running into the error.
If I enclose the ‘using namespace …’ line in a “#if PLATFORM_UWP … #endif” block, it will build in VS2015, however I run into the same error(s) when packaging.
I’m currently rebuilding everything (mainly because of superstition), I’ll let you know if the issue resolves itself.
How might I go about adding the winmd reference manually in the meantime? Thanks again! You’re my hero @ 
Update: my superstition didn’t pay off. It is behaving the same. With the Whitelist entry added, we see that the plugin is now being built for UWP:
MainFrameActions: Packaging (UWP (x86-32bit)): UnrealBuildTool: C:\Users\Documents\Unreal Projects\HoloTest\Plugins\AdasiEmptyPlugin\Source\AdasiEmptyPlugin\Private\AdasiEmptyPluginRender.cpp(21): error C2871: ‘Holographic’: a namespace with this name does not exist
MainFrameActions: Packaging (UWP (x86-32bit)): UnrealBuildTool: ERROR: UBT ERROR: Failed to produce item: C:\Users\Documents\Unreal Projects\HoloTest*Binaries\UWP32*\HoloTest.pdb