Campaign managers that are doing this intentionally would have the option to ignore this feature, though. If some managers started using it, and some didn't, it might actually make the trap more effective, by lulling potential participants into thinking: "No banner? It must be safe to apply then..." 
You may not want to use the word trap here. You may want to use better wording
We don't want anyone to fall in the trap. Right? Your patch will help them to see if there is any important announcement. So, it will help them to notice important things. It would be awesome if managers could also use this feature in their old threads. For example, We cannot turn our self-moderated threads into normal threads. Also, We cannot turn our normal threads into self-moderated threads. If the new banner system works the same, I don't think managers would create a new campaign thread to use this feature.The fact is that so many users who apply in a signature campaign doesn't care to read the instructions of signature campaign before get applied, they only concentrate to know the specific rate campaign is to pay participants and the board to post, majority participants that's not been selected in same campaign is bumping and reapplying in same signature campaign, another thing is that some has seen the instructions but they keep on trying to see if they will be lucky to be select. But cm state it categorically that one that bump it's application will be ignore, you will see that no user or application that will be reapply or bumped
Look at the signature Campaign I and PowerGlove are in. The Campaign manager said Apply if you are a prominent forum member or an ex-ChipMixer participant. People started applying there with one merit earned in the last 120 days. I don't think any prominent forum members earn less than 100 Merits in the previous 120 days unless they are inactive. The campaign manager said reapplying is not allowed, and they will be put on the ignore list. But who cares?