Here’s the code I created.
https://github.com/theideabureau/acf-reusable-group-field
This plugin works well and is very handy. However I’ve just run into one caveat – when using the Reusable Group Field within a Flexible Content Field you can only use it once per page.
If you try and use a Reusable Group in more than one row it doesn’t seem to store a separate database entry for each instance of the group.
This would be such a great feature to have as it would give ACF a really nice modular approach.