One option would be to create your "pick your measure" calculation and parameter and set it to the desired measures, as normal. Then, create a second calculation that sets speed and volume to zero so that the reference line for these would be set to zero (not ideal, but it might work for you!). The second calculation would look similar to the following:
case [Pick the Measure]
when "Density" then sum(Density)
when "Speed" then 0
when "Volume" then 0
Then, place the original calculation on the rows/columns shelf and the above calculation on the level of detail shelf. Finally, when the reference line is added, make it in reference to the new calculation.
Hope this helps a little bit!