I dont think that is possible directly from C++ with the tools and templates that have been provided to us, as I understand the html5 interpreter reads from the bytecode file which is generated by the compiler which we do not have access to the source and that would be tier1 code anyay
what is needed is a template that compiles the C++ directly with emscripten and looking through the files in both the user Tier2 folder and the AGKLibrary source I do not see any so I think the HTML5 support is strictly for Tier1 projects.
Unless I am missing something or someone writes a wrapper, I dont think its possible as it stands.
EDIT: it is posssible to setup Visual Studio to compile emscripten projects you just need to install the required files, maybe you can then import the AppGameKit code into that but its not something i have experience with ..... try youtubing it