I always thought the C-o subshell needed a status bar or something. Not once did I find myself launching mc again because I forgot it was already running. Luckily it warns you when launched from the subshell.
mc sets $MC_SID in the subshell, so you can query that in your prompt to mark prompts running under mc.
Another option might be to bind F10 to start mc, so that it acts as a toggle whether in mc or in a non-mc shell. You'd need to wrap a zle widget in zsh, or add a '$if Bash' guard to your inputrc with bash.
Another option might be to bind F10 to start mc, so that it acts as a toggle whether in mc or in a non-mc shell. You'd need to wrap a zle widget in zsh, or add a '$if Bash' guard to your inputrc with bash.