Smush custom image sizes

We love using WP Smush Pro but in order to not fill our servers up with loads of images we don't use add_image_size, rather we create custom image sizes on the fly using a method similar to this:

The resultant images are saved in the WordPress database just like the others, but it seems that WP Smush Pro doesn't look at them and so doesn't smush them.

It would be a big help if you could update Smush to smush those images! Is this possible?