I can see a big downside to this.
Many codingame users just don't know about the manual mode (and i wonder if this feature is hidden where you are not in expert mode).
So, let's say you are a noob. You click on "Send parameters to IDE". Manual mode is activated but you don't know what it is. Do you see the problem ? Every game you'll play will be with the same seed and you won't know why. Even worse maybe you'll just not notice that.
So my point of view: I agree with Bob suggestion, but only if the manual mode is moved somewhere else in the IDE. And by somewhere else i mean somewhere bigger and more accessible.