I wonder if they have a hook at a higher level that could affect this. The locate_template function does not actually include any PHP template files by default. Since the $load argument defaults to false it should just be returning the template file path.
If there are no better options, you can just mirror the plugin and make the change. If you’re deploying with bedrock then you could host it on a private repo. This should include some sort of auto-update and notifier for changes.