Does this mean if someone were to somehow get the message while wearing five completely different sets of gear, they would then have a negative modifier on every item in the game? (-1 on everything worn during the messages, -5 on everything else)
The term "set" is simply used to denote the gear you are wearing when you get the text. So it is impossible to wear things from different "sets" at once when you are in fact wearing one set. So for instance, imagine I am wearing:
Pilgrim's Hood
Churburg Cuirass
Light Leather Boots
Mail Gauntlets
And I get the chadztext. These items are now:
Pilgrim's Hood +3
Churburg Cuirass +3
Light Leather Boots +3
Mail Gauntlets +3
And -1 on everything else (whether its everything in my inventory or every item in existence, I do not know.)
Now lets say I got really lucky the next day and got it again, but I was wearing
Nikolosjoe Helm -1
Light Mail and Plate -1
Light Leather Boots +3
Mail Gauntlets +3
Now my items will look like this!
Nikolosjoe Helm +2
Light Mail and Plate +2
Pilgrim's Hood +2
Churburg Cuirass +2
Light Leather Boots +6
Mail Gauntlets +6
You can see how wearing a variety of gear will help raise all of your numbers eventually. If you are interested in being your clan blacksmith, you will want to pay attention to your gear craft bonuses and wear a good mix of items every few rounds (or every time you get the text.) Here's hoping there will be an explicit list we can use for reference that tells us our personal bonuses to craft each item; tracking it in your head or trying to manually calculate it all will be a huge pain in the you-know-what! Especially if you missed a message or two for whatever reason.