First of all, you state you are using LT; LT does not have the capability to utilise LSP functions without the aid of some form of add-on, such as LT Extender.
One way to achieve the results you desire, would be to create a block which contains no objects; then create a new Dimension Style.
Within the newly created Dimension Style it would be possible to set the "arrowhead" to be the block you created earlier. This would then give the impression of there being a gap at the ends of the 'line'. Then create dimensions, instead of lines between the column positions