A little late to the party, but I just became a mod on safenetwork.io, which uses discourse.
Another drawback to the workaround is that there is no way to link to the flag, or otherwise directly reference it in a PM. This can become problematic when old flags need to be referenced, as they are not able to be linked to.
Also, if one of my fellow mods closes a flag, I have no indication what was done - the flag itself does not specify. Adding the PM option would allow resolution descriptions to be posted only if necessary. Most likely this would be easier than adding additional options of how the flag was handled directly on the flag page.
The reply option to flags if they are opened with “something else” is what I see the template being most effectively set up as. Adding a “reply” option that PMs “@ moderators” would alleviate both problems, without introducing auto-spam to moderators, because the option to create the PM would be available on an “as-needed” basis.