You mean you do not ignore cease and desist orders by principal? Why? There should be at least some criteria by which you judge which ones you can ignore and which you can't? What if they told you to cease and desist programming forever on any project because they state in their terms that once you mess with their UI with JavaScript you are not allowed to program anymore? Ridiculous right? So would you listen? Probably not.
So lets see what they are 'forbidding' you: They don't want you to write code that makes browsers do other things to their site... How is that any of their business?
What if a new browser comes along that renders all of their fonts differently so that they become unreadable, is that any of their business or is it the business of the people who use that browser? I'd say the latter.
I agree that a disclaimer and a change of name should be enough. I'd suggest 'SlackingOff' as a name.
You are free to write any extension that does anything to any website as long as you aren't hurting users' human rights and as long as you aren't hacking them if you ask me. Users can decide whether they want to use your extension perfectly fine on their own.
Also note that you are free to not write any extension as well. It is your life of course :)