In some journals and more frequently for letter format publications, authors are encouraged to incorporate extra details on materials & methods into a supplemental section.
It would be useful (but not imperative) to provide an option (like a checkbox) upon inserting citations that distinguish them as supplemental or not.
This way, you could populate two sets of references when formatting citations instead of creating them in separate google docs and pasting in right at the end. Not a huge deal but just a thought.