Would this be always a Leader specified by only the arrowhead location and one other point [or, always the same number of defining points if other than 2], or would the number of defining points need to vary?
It shouldn't be hard to do something like that if you don't mind not seeing the length value text until after you've given it the other end of the Leader. [It may be possible to get the text in and then let you drag it around, but I think it would be a lot more complicated.]
Over in the plain-AutoCAD Forum where you first raised the question, it was about a Line, but here it's about a Polyline, in which case another question arises: Should it show the length of just the segment you pick on, or of the entire Polyline?
You also talked about picking the line, then picking the point on it where you want the Leader arrowhead, but that step is not in the current description. Does that mean the arrowhead should go at the point where you pick on the Polyline?
Kent Cooper, AIA