Many plugins use /wp-content/custom-plugin-folder/ to store customized plugin data (WPTouch comes to mind).
Just use the constants WP_CONTENT_URL and WP_CONTENT_DIR Docs to check for the existence of your folder and retrieve any available skins.
The following article, although not directly related to this Question, explains the importance for plugins/themes to search for translations first in the wp-content/languages folder before loading its own packaged .mo files. It’s a worth read and hopefully you’ll apply the concept in your next release 🙂
Loading WordPress language files the right way
http://www.geertdedeckere.be/
I would like to point out that is important to load custom user language files from WP_LANG_DIR before you load the language files
that ship with the plugin. When multiple mo-files are loaded for the
same domain, the first found translation will be used. This way the
language files provided by the plugin will serve as a fallback for
strings not translated by the user.