Saw this posted to the Lycoming group, and it sounds like a pretty good idea. Anybody heard of it?

"A friend flying an IO-360 powered Glasair installed an oxygen sensor
system that has an panel mounted indicator light to show when balanced
combustion is reached. He simply adjusts mixture until the light comes
on and knows he has complete combustion (his theory is that EGT is
really just a good guess). He has had very good results from this
system in terms of fuel economy and performance.

I believe he purchased the system from Wag a few years ago. As I
recall it was for experimentals only. They do not currently list such
a system in their catalog. I have not found one at ACS&S or Chief either."