You should be required to add a 'however' premise explaining how a post is fallacious when you report it as such.
This would add clutter to the board and a logical fallacy is best given the premise 'but' to represent it being an objection. I would propose a 'custom' fallacy type that allows a comment that would be visible only when clicked.
But I still think that it should be mandatory for every fallacy reported. Simply because
a.) people dont seem to know what some of these fallacies are and report them anyways. b.) the person being reported should be given an explanation so they can understand how what they said was fallacious.
The clutter could be removed if the fallacy had to be clicked on to expand/collapse either a popup window or list item to display the reasoning. They should also implement a way to expand/collapse trees and arguments to allow more friendly viewing.