Just to clarify what your objective is, rather than imagining one possible solution for that objective, some questions:
1. Do you need to know the entire length of a Cable object? Or just part of the Cable? What about the length of other objects?
2. What do you want to do with that information? Display it on the drawing? In the Cable Schedule? Anywhere else?
3. in what drawing types is this a requirement? (Block Schematic? Rack Layout? Panel Layout? etc)
In other words, what is the problem you're trying to solve?