Implementing powerline communication on a high voltage DC line with an AC power-line adapter
Generally, yes, this should theoretically work.

Now, the PLC channel as seen by commercial off-the-shelf PLC modems is a strongly cyclo-stationary one, with long (in terms of communication technology) periods of bad signal conditions, which the modem has to work around, which is why they typically contain a lot of logic to estimate the grid-voltage-periodic behaviour of the cable. In your case, that 50 Hz (or 60 Hz, wherever you are) period doesn't exist, so these estimators might run amok. That might put the modem in a continuous "worst case" mode. I don't know. I don't think  behaviour on a DC line is specified for any of these modems.

However, things probably work. You should try.

However: 

PLC is pretty annoying, to be honest. Not a medium I ever enjoyed working with. Your data signal is competing with the noise of multiple motors, probably BLDC. That's the worst-case scenario for PLC.

Now, you, by definition of what a tether UAV is, pretty much always have something that is a line of sight, or nearly a line of sight, between ground station and vehicle.

Simply use radio comms. By now, they play at least in the same data rate regions, they tend to be cheaper and lower in power, and they (probably) add less weight to your UAV. From a reliability point of view, it's probably far more reliable than PLC with a lot of motors on the same line. 

5GHz ISM-band Wifi would clearly be my prime choice here. The fact that you happen to have a cable running alongside your Fresnel zone will actually probably even help.