Where to store asset backups for plugin?
The only thing that Craft guarantees to be writable is craft/storage (and to a limited extent, craft/config).

But, 99% of things in craft/storage can and will be rebuilt as needed depending on the request, even if the entire folder has been nuked.

You can either work within that context, save your assets as blob's in the database (would not recommend) or in your plugin's installation instructions, make sure that you specify they create a craft/plugins/pluginHandle/writable folder you can save your asset revisions to.