Is there a token for a gallery's cover image (or n-th image)?

I'm in the process of setting the metadata fields for my pages using the metatag module. This module integrates with the token module to automatically generate the relevant meta tags in the HTML head.

In the case of a node gallery page, I'd like to use the cover image to fill the "image url" meta tag. However I can't find the corresponding token in the (very, very) long list of available tokens.

Similarly, I'd like those gallery pages to generate tags for Twitter cards of type "Gallery". In that case the required fields are "1st Gallery Image", "2nd Gallery Image", etc (up to 4). Each field is supposed to be filled with an absolute URL.

Is there a way to do that?