It would be a lot easier for the template designer, if the existing template (without modifying the wiki parser!) could link to an intermediary page called Special:Serialsource which could transclude text from a user-editable [[Wikipedia:Serial sources]].
Some way to access a post/get value in a page would do it, then you could just link to Wikipedia:Serial sources?issn=12345678. I can imagine hundreds of potential security issues with that, though, so it would have to be done very carefully. It could have lots of uses, though.