Sorry that that's not much help
I couldn't see anything in the books that even comes close to replicating any sort of line-thrower or harpoon gun. Even the one thing I thought would certainly have some kind of special rule, the 140mm RAW, is treated as basically a rifle grenade and thus comes under Heavy Weapons skill and the usual indirect fire deviation rules etc. etc.
Nothing there seems to come close to usable for working out game specifics for the particular characteristics of a line-thrower/harpoon gun.