How can I have one circuit always utility powered while all other circuits are either generator or utility powered?
The THP108 PowerBack audible alert might fit your need. It is a battery-powered alarm which detects mains power via an antenna wrapped around one of the supply conductors (upstream of the main/interlocked breaker). There's no electrical connection to the conductors. I don't have this device (yet?), but from what I can tell, a person switches this thing on while they're running generator power, and the alarm sounds whenever utility power comes back on. (photo from Reliance Controls)

Still, though.. I'm interested to see whether one of our expert electricians will come along with a suggestion for tapping the supply for an extra panel as described in your question, or perhaps a code-compliant means for adding a neon (or other) pilot lamp that indicates when utility power is live.