as the rules stand at the moment I do not believe you can do that and to be honest I agree with these rules.
If people start to have massive lines in the sig it makes them look stupid. I personally ignore such text.
Some time ago I actually changed my settings to stop showing sigs as I was bemused by the amount of people who were posting very short posts but had the maximum (if not more) space allowed for sigs taken up, I was finding myself just scrolling past sig after sig.