I was wondering if it would be possible to password protect hassio’s panel within the HomeAssistant frontend.
Do you mean to protect the general web UI? Then just set the https://home-assistant.io/components/http/ the api_password and dont whitelist (trusted_networks) your local network.
I dont think that this would be possible without some major changes in the frontend. You could copy the existing component of the panel you want and implement your own security auth.