None of PyPI, npmjs, or conda-forge provide a canonical way to specify an icon, so it’s probably github or the highway.
Further: I wouldn’t burn too much effort on the 3.x extension manager, as when it’s not broken, it’s at least very slow, and requires nodejs… you may want to weigh in on this for the 4.x line, as it will be more driven by the constraints of multiple package managers, and extending these is something that could happen, e.g.
package.json#/jupyterlab/iconUrlpyproject.toml#/tool/jupyterlab/icon_urlmeta.yaml#/extras/jupyterLab/iconUrl