The code is simple to implement. That is not why it hasn't been added.
My concern here is that we have gone from a state of play where the argument was "the GUI is too cluttered with preferences... stop adding them!" to a position where we add the preferences without the GUI component of them. With things like chat PMs beeping, for example, I'm not too concerned since it is a niche issue. With mana burning, I feel that if it is a real concern we maybe need to think more carefully about its implementation.
How often do people really want to go above or below 1,000 (+ adv)? I wouldn't have thought that below is really a concern since a) what else would you do with the mana? and b) buffs are balanced equally, so there really isn't much mafia could be doing that's harmful there. If you are happy with the turns built up, you would probably turn burning off. So really we are looking at folks wanting to go above 1,000 + adv. Which leads me to the question of how many people does this effect?
If it is a decent proportion of the user-base, maybe we need to look at a better presented setting.