Damian:
Firstly, I know for a fact that you all in the big bad mod corner have records galore. You can easily shoot a message around the team that says, "Hey guys (and girl if CF is still active), I just sent a message to X about their post in threadID=X which I thought crossed a line. If anyone thinks I misjudged, let me know so I can offer some sort of explanation to X and so that I don't make the same mistake in the future." That message gets recorded because it goes from personal email to mod email, and all of those are saved, or it can be written down somewhere if that email approach doesn't work. I can't give you specifics here because I don't know how you all operate, but I absolutely assure you that, given the organizational system that kestas and friends designed over the years, you will have no problem keeping track of who has been warned and for what.
Secondly, you might be confident that the line you've drawn is right in line with other mods, but clarification never hurts. You have your own ideas about these things, and maybe one day you're feeling a bit more irritable and you go after a post that usually wouldn't be so bad, but today's just a shitty day, you know? So you have a chance to go talk it over. Besides, every other person you contact is going to ask what Zultar, Jmo, or Abge thinks about it, and since you've already talked with them as well as other mods, you can relay that information quickly and resolve the situation quickly.
Lastly, I really think that PM warnings (or email - both would work the same) will lessen the "indignant" side to your job. You can talk on a more personal level and you can have civil back-and-forth conversation, whereas on the forum someone else that, let's be honest, has absolutely no fucking clue what's going on is going to jump in (probably me) and give some dumb analysis of the situation that makes you out as the villain and blows the whole thing up. When you talk one-on-one, you're not going to get ganged up on, but when you open it up to everyone, you're almost always going to have to deal with backlash and in some cases a permanent grudge which has many players right now calling "mod abuse" on a regular basis. If you talk privately, personally, and civilly, you'll still run into this, but not nearly as often.
While this may be a terrible analogy, I think it's worth considering. If you break a rule in game, the moderators never jump into the global chat to give a warning; it's typically by email so that you guys can record what's going on. You also keep other players out of the picture and that's a good thing. With PM or email, you can take the same approach toward the forum - easy to record warnings and keeps the public backlash out of it.
If something happens and a player continually disagrees with your judgment, they can still appeal to zultar/kestas, and if the mods decide that we need a public discussion about a rule, we can have a public discussion. The difference will be that in that case, the discussion won't turn toward a single incident like this one did because it won't stem from a single incident; rather, it'll be broader and about the topic at hand so you all can get an actual idea of what the community wants, not just what we think about a given situation.
Hopefully that all makes sense, but it's late, so if something's weird, I'll try to clarify.