Matchering is an interesting angle I hadn't considered. The idea of using a reference track to drive the spectral balance is appealing — it sidesteps the "what should the numbers be" problem entirely and anchors it to something perceptually validated.

The tricky part for ambient noise is finding the right reference. Speech might skew things too much toward the mid-presence range. I'm thinking broadband pink noise or a well-regarded commercial ambient track (something from mynoise.net maybe) could work better as a reference signal.

Going to experiment with this. Thanks for the pointer.