Then there is no direct way.
Some calculation will be required or you could use the measure command to try and create a variable from measuring the total length of the centre line elements.
there might be also another way to get it.
You can have a variable calculating the Volume / Area (cross section)
Both can be read out directly from the model and so any change of the model will be reflected in the Length = Volume / Area variable
The volume to length thing does not account for stretching, that assumes K=0.5. He will need to find out the K factor for the tubes on his tooling to have a chance at getting it right. Then use the square version (insribed and excribed) to start laying it out. Then combine the two for a manual layout. Not fun, but can get it done right.