Would it be possible to change Metabolic Generator to use saturation first and hunger second, like sprinting? Or at least add a config option for it? It wouldn't waste as much food when one is standing still.

No update for 6 weeks, so I'm assuming everyone is happy with current values

It was already changed to use exhaustion, here: 5cd1fae
Currently, the metabolic generator add 6.0 exhaustion every 54 seconds (about 0.0055556 per tick).

Here are other values, per action unless otherwise stated:
0.0 for Walking or Sneaking (per meter)
0.001 for Swimming or walking in water (per meter)
0.005 for Breaking a block
0.006 for Metabolic generator (per tick)
0.05 for Jumping
0.1 for Sprinting (per meter)
0.1 for Attacking an entity in close combat
0.2 for Sprint-jumping
6.0 for Healing 1 heart