I did something similar in my jeep until I sold it. I had a fuel cutoff button and an alternator field switch, an a third switch.
I had it so that when the fuel cutoff switch was depressed and the third switch was engaged, the A/C would kick in. I used it with engine braking and it worked marvelously. The 2.5 litre 4 banger didn't have all TOO much engine braking, so you could feel it if you engaged/disengaged the A/C while engine braking.
I also had a diode so that when the A/C came on, the electric fan came on. Another diode so that if the electric fan came on, the alternator would come on. The diodes made it so that I could turn on one with its dependencies and not the higher-ups.
You could do the exact same thing with just the alternator, but I would recommend adding a diode between the electric fan and alternator field relay. That way you won't be as likely to drain your battery when your engine gets hot. But you probably don't see too much electric fan use because as an eco-driver you probably are either moving or have your engine off
Unfortunately, with the cool down time of my turbo, I am EOC impaired