I attempted to override the template behavior.js file per documentation and it doesn't appear to work.
can anyone confirm if it's working?

Could you be more specific please and provide us with a link to your gallery?

according to the documentation here:
www.joomlaworks.net/content/view/51/41/

you can override a template's js file via your joomla template html directory:

You are able to override all files or selected files, as you would with any other MVC component or module, with the better exception that Simple Image Gallery overrides CSS and JS files as well, as long as you maintain the same names (template.css and behaviour.js).

the php template override works fine via the override, but the behavior.js file does not override as expected.
my site is in development, so it's not available publicly. but you wouldn't be able to troubleshoot this issue from the public facing side anyway.