Dear @aoverton ,
I usually set up intellisense with ROOT libraries and it works. I believe this post is relevant in the discussion.
In particular, something else you might want to try is to place the path to compile_commands.json
in the c_cpp_properties.json file. This should be found in the build directory. I am unsure though about its presence in the CMSSW environment.
Cheers,
Vincenzo