It seems that according to part 107 a visual observer can be used to maintain a line of sight on the vehicle, as long as the PIC and the VO are in communication. It also seems that there is no limit on where this VO can be, as long as a VO or the PIC always has their eye on the vehicle.
I can't see why you couldn't operate long distances with VO's used as "stepping stones". I'm not a lawyer but it seems clear in 107.31 and 107.33 that this should be entirely possible. Any thoughts, or are people doing this and I just wasn't aware?