Repsonsive email templates in eNewsletter

Now eNewsletter has a fabulous number of filters for practically everything, except as far as I can see the <head> tag contents.

I and looking at creating my own custom template and want to use a responsive layout so need to add a viewport meta tag in the <head>.

This would be neat by using filters in the templates functions.php, but that requires the plugin to be updated to include that filter.

Any chance?