You can do that already, just cut down the amount of characters allowed in a signature so they attract less attention.
Bad idea IMHO. Signatures are used for personal advertising too without much designs.
But wouldn't that result in a dramatic reduction of the payout? Because the campaign company would be getting less views.
Only if they don't check and ban people from their campaign correctly. Some users might disable with keywords such as cloud mining but I don't think it would affect much.
-MZ