From here, it appears that Issues Moderators have the ability to add badges to nations. (Again, I do not know this for sure) So I believe that in lieu of a technical solution (to save admins time), the moderators could simply apply the badge to any co-author whenever an issue gets published, in addition to all current co-authors without the badge. As issues co-authors aren't as common as those in the WA, this would not be a very frequent occurrence, so moderators would not have to waste too much time doing this all the time. I do not even believe that creating a special 'Issues Co-author' badge is necessary - since Issues Co-authors are credited less like co-authors but more like joint-authors (Authored by Nation X and Nation Y, as opposed to Authored by Nation X, co-authored by Nation Y).
I believe this protocol will be a good idea as many issues co-authors often put as much work into the issue as the author, so it's only fair that they get the credit to. For example, Jutsa has often said that they believe that their issues co-author Australian rePublic should had gotten the badge for the issue instead. This change may also encourage more people to help co-author issues.
I hope to see this, or a similar change, implemented! Thanks for reading!
Appendix A: List of current issues co-authors
Tishanda Issue#628
Zwangzug Issue#700
Australian rePublic Issue#895
Baggieland Issue#1041
Pogaria Issue#1059
Tinhampton Issue#1115
Caracasus Issue#1119
Jutsa Issue#1137
Canyamel Issue 1268
Australian rePublic (again) Issue 1426