Thank you for the explanation.
Your scenario cannot be accomplished exactly as described, but you can achieve the same behavior (i.e. a horizontal line) if you nest the crosstab in a panel item and set solid top border with desired width and padding. This way the panel would stretch itself according to the crosstab. Of course this is only applicable if your case allows to have the horizontal line "moved" to the same section where the crosstab is.
the Telerik team