First post and new to the forum. I'm the type of person who is willing to take a methodical plunge if I want to change or alter something - in your case shiny nickel hardware on a nice jacket.
I have fair amount of experience modifying copper or brass to age or patina (not on jacket hardware). I've experimented with all sorts of home brew concoctions with dismal results. The Finishing Network has a wealth of information on subjects such as this. The downside is you'll get recommendations that requires chemicals that are not easily obtained. There are companies that make products specifically designed to do exactly what you're wanting to achieve. They've taken the guess work out and the products work as described.
Someone else pointed this out already - if any of the hardware has a clear coating chemical aging will not work.
I have a few observations based on some of the info in your post.
A. Schott C.S. told you it would not age much with time. This probably means one or two things.
1. The hardware has a very good quality nickel plating.
2. The nickel hardware has a clear protective coating - I doubt this is the case.
If I where you and really wanted to do this I would call Schott again and ask them which of the above apply. The zippers with the Schott logo are made by Lenzip. Again if there is a clear coating I would not attempt to age the hardware. If not then I would go for it.
Back to specific products. The is a product called Nickel Ager. I've used products from the company that makes this and they work. The directions on the product call for the items to be dipped, you don't have to do that. You can apply with a Q tip. Then use a spray bottle with water to rinse after a few minutes (1 -5 min.depending on the level of aging you want).
I would do it this way (referring to myself if I were to do this)
1. Determine for sure if the zipper or hardware is clear coated. If not i wouldn't hesitate to age the nickel.
2. Decide if your willing to spend $10 -$20 or so and dedicate 2 -3 hrs. of your time to make this happen.
3. Order aging chemical and find a like nickel test piece and test to see how long it takes to get the desired patina you're looking for. Then test on a piece of like leather. Carefully applied and rinsed I doubt the leather would be effected at all. Chemicals that effect metal/coating generally have no effect/affect on other unlike materials.
3. There is no doubt this can be done with excellent results with the proper chemical, careful application and light spray bottle rinsing.
I've attached a photo of the "Nickel Ager".
Life to short to not get what you want!
2.
Wow, what an amazing post. Thank you very much! Any chance you have personal before/after posts with this method?
How does this chemical react with leather and dyes?