Just to complete the circle:

This issue was pursued through the Conda Community Forum and found a solution there from a Community member:
https://conda.discourse.group/t/conda-build-throwing-warning-error-http-404-not-found-for-url/661

I never was successful with installing the conda package for the jupyterlab-hide-code extension that I had built with whl2conda. I eventually tried the conda-pypi package (still in alpha) mentioned in the forum above, and it worked: jupyterlab-hide-code is now installed in my project's conda environment. Be forewarned, though: using conda-pypi will internally mark all of your conda environments that are outside of the base \envs folder as Externally Managed. I hadn't quite understood what to expect in that respect, and the results briefly caused a few heart palpitations. conda-pypi tracks all those environments and explicitly makes pip installation inside of them a last resort method -- lots of warning messaging. Best to read the conda-pypi documentation thoroughly--and the post above-- before you try it.