They'll have to come up with some kind of a simple signal to avoid confusion with other cables. ISDN presents a similar problem i.e. carried on a phone line pair, but without a dial tone.
Surely some narrowband signal could be applied that would not take up too much bandwidth yet be used to ID the line.
I don't think DSL customers are going to be too happy if they have their service interrupted randomly by linesmen/women accidently disconnecting them!