Neat trick! Thanks for posting.
not exactly DIY ready for apartment dwellers, but interesting nonetheless. Obviously (to the old timers) you can get pretty nearly the same exact result by boiling or steaming, and yes, it does work for guitar woods. Main disadvantage to steam is a short open time. With either ammonia or boiling, I'd imagine there will be problems with cupping. Steaming has some limited use still IMO. Works for troublesome wood binding. Steam, clamp to form, and then bend them on a hot pipe.
RE darkening: Yeah ammonia will change the color, sometimes quite severely! I use it intentionally in preference to stain. If you can get the 'janitor' strength ammonia, it works pretty well. Ace hardware has it. For small things like guitar necks (etc), I find it easier to wipe the wood with ammonia directly rather than to set up a tent and fumes.
Different woods react differently, but I really like the dark color it imparts to mahogany. While it's something that be used with caution, it isn't anywhere near as poisonous as potassium dichromate.