Mod allows to set custom skins location url on the server side.
Mod uses "http://skins.minecraft.net/MinecraftSkins" path by default.
Specify ssskins.url in the server.property for changing the skin url.
All skins for each player should be available on the path "<ssskins.url>/<playername>.png"
This mod does not require modification of the client.
It's enough to put this mod on the server side.
Questions and discussions can be made in gitter.
If you need to build a mod for a specific version of minecraft, contact me via gitter or in any other way.
UPD:
I'm afraid that since Feb 2018 the http://skins.minecraft.net/MinecraftSkins is not available anymore.
https://bugs.mojang.com/browse/WEB-985
You should maintain your own server with skins now.