This is interesting, because I'd just read
an article in a woodworking magazine about how Greene & Greene would use a similar approach to darken wood.
They made two interesting points in the article:
1) It's the ammonia fumes that do the work; just spashing the ammonia on the surface won't work, because when it's in solution it has a different chemical composition.
2) If you're doing wood, expect it to take up to 36 hours, assuming you train a heat lamp on it to keep the temperature inside your plastic tent to around 80F.
I'm tempted to give it a try on some of the walnut stripwood that I got with my kit that's significantly lighter than the rest. The article talked about managing an uneven wood color, such as some light walnut sapwood, and suggested "painting" the lighter areas with tea before fuming with ammonia. The ammonia darkens the wood by acting on the tannin, and a tea wash on the lighter wood adds more tannin.
Just be careful with the ammonia fumes. The photo with the article showed the woodworker wearing a gas mask.