Inserting JS snippet in the footer if there is no wp_footer?

I wrote a plugin which uses wp_enqueue_script() which will include the snippet in either the header or footer depending on the parameters you pass it. The in_footer parameter works great – if the theme utilizes the wp_footer() hook.

What are my options if I want to include the JS snippet directly before the </body>tag if a theme is not using wp_footer() hook?

If the theme does not implement wp_footer() a lot of functionality that depends on it will break. For this reason, implementing wp_footer() is required to get your theme accepted in the WP.org directory. So it is not unreasonable to break when wp_footer() is missing.

Be sure to add a notice in your FAQ that you require wp_footer() in the theme. You can also add a check in your plugin admin page: checking for the existence of wp_footer() can be as simple as loading a page with a special $_GET variable in the background, which will trigger a hook that outputs some control code in wp_footer, and reading the result to see whether this control code can be found.