Whilst this is not really it's intended purpose, I have a macro under development that will display the total length as a polyline is being drawn (but not as you move the cursor) provided that the polyline is all straight edges (no bulges).
The macro is an Offset macro, but it could be used to draw a polyline from the points entered by setting the offset to zero. It is currently in development and in it's current form will only draw chains of 2d lines or vertical polygons (there are a couple of polyline options on the menu, but there is no code associated with them as yet).
The polyline options should be finished in the next few weeks and the final macro released then. In the meantime you could look at it using the 2D line option to see if it would fulfil your need. It is a Delphi macro (dmx file) and the current beta can be downloaded from http://www.dhsoftware.com.au/downloads/Offset.zip
if you are interested in looking at it (I would be grateful to receive comments/suggestions from anybody that does look at it).