AFAIK there is no fail safe way to implement this into all available themes through a plugin, and IMHO, I really think this is more theme territory than plugin territory, but as I said, that is only my opinion.
The big problem here is that no plugin (not even you) will ever know what markup or hooks, filters or functions is used by a specific theme, let alone know if that specific theme even has wp_footer included.
Your best available hook is the wp_footer hook to add your custom copyright text.
I would probably not worry to much about this, because, let’s face it, who is going to install a plugin to show copyright info if the theme they are using already has that feature included, and if they need to extend this, it is probably better to make use of the filter provided by the theme author. So most probably, someone that might download your plugin is someone that might not have this feature included in their theme
A preferable counter option will be to ask the end user to add a specific hook that you’ve created to their theme where they need to show this copyright text, basically the same idea that a plugin like wp_pagenavi uses to add pagination to a theme. This way, the end user has more control over placement. TBH, I think this is the way forward for something like this.