I guess it would be a custom build. I don't think you would ever have more than a few different lengths. I only have one "spare" line that has an extra liquid disconnect on it.I keep my temp constant and have a manifold with three mini regulators on it. So when I have a beer that I want to carb at appreciably higher or lower volumes, I can adjust the psi and then swap out a liquid line to get the pour right. It's overkill but I'm a sucker for flexibility.
I'd probably do OK if I just slapped 10 footers on all the kegs and just dealt with the slow pour.
Todd