Are they implemented into the mod for players in any way?
Also, do they work with the Elytra wings?
I'm asking because I have a mod using the same library and I'm interested in your 1.12.2 execution.
Thanks!

The capes are only on players I add in the config file, not accessible to other players.
It does change the Elytra, they are taken from the same cape texture by the game.
All it does is inject a custom texture url into Minecraft's built in cape system.

That's pretty cool, thank you.
I don't know how you found the updated field for 1.12.2, but do you mind if I apply it to my mod as well?

Yea that's fine, it shouldn't clash.

I noticed that you're using the same field value I was using for 1.10.2 actually, but I still don't know how you work with the Elytra using the DeveloperCapes code.
The cape works fine, but whenever I wear the Elytra the texture turns to white:

Does this not happen to you?
(Also by the way, if you're still using player names you might consider changing to UUIDs: jadar/DeveloperCapes#43)

The Elytra uses the same texture as the cape, you must just have white where the Elytra should be. I think if you have a cape it will always override the Elytra texture.

You're right. I added plrTxtrs.put(Type.ELYTRA, location); and changed the cape file to include the Elytra and it worked, later I removed the extra code and it still worked, so the problem was just my cape file. Thank you!