If you create the tube using Frames or XpresRoute you can get the length of the tube
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.