That's Caustic Soda (Lye / NaOH) you're thinking of for saponification. NH3 isn't basic enough for the process to work as described.
Just me I get the racks out and soak them in hot water and clean them before the stuff on them sets like concrete by application of a bunch of elbow grease and a pot scourer.
I like the idea for ammonia on oven racks and gas BBQ grills. The bag it idea sounds easy. Now about the innards of stove or grill.
As Phillis Diller once said her oven was so dirty she could only fit in and bake one cup cake at a time.
I know when it is time to clean my oven is when I have to bake something at 450 for an hour and when I open the oven the smoke alarm goes off.
Way way back in the 1950's when my parents build their first home, Mom had some novel ideas for the time. The cook top was on the counter and the oven was built into the wall at eye level. No problem cleaning the oven just wipe down.
Today 50+ years later we have gone back to the old designs of cooking from the wood stoves, one unite, cook on top, bake on bottom.
Cleaning a oven means one must just lay flat on the floor to clean the innards, the oven door opens downward and to clean, one has to crawl over the door to reach the back of the oven.
I have yet to see a side open door to a one unite stove, much safer as accidents do happen when one leaves the oven open and a running child can fall over it.
My microwaves door opens to the side, my refrigerator doors open to the side, all the doors in the house open to the side, so why does my oven have a pull down door that causes one to bend over to fill the oven or remove very hot food??????
Nuts, when this stove goes kaput, I will buy the separate oven and cook top, never again have to crawl into a nasty oven to clean it.