Hi Lomax and all:
As Douglas said above, you can hang pretty much anything you want off of the tether without affecting the Homeplug adapters, as long as you isolate it from the RF signal of the adapters through the use of a choke / ferrite / inductor or whatever. If you surf the web for data on HomePlug AV (that's the protocol we use), you can find it's frequency band.
With regard to what happens to a DC/DC converter when you drop below the minimum input voltage: you have to look on a case-by-case basis, but most will shut down. Look through the documentation for something called under-voltage lockout (UVLO). That's the thing that shuts the converter down.
I agree with David above that the best way to deal with tether power is to have it charge an on-board battery pack, which then handles the peak current loads of the motors. This does add some complexity, but gives the most flexible overall solution. Last spring I had a design for this sketched out for OpenROV, but it got put on the back burner when we did some tank testing of motor/prop combinations and realized how inefficient the props we were using at that time were. Once we switched to the Graupner props, the battery life went way up, and we decided that power-over tether was not as important as some other things that were brewing at the time, such as switching the low-level I/O architecture from an Arduino Uno to an Arduino Mega.
We'll probably revisit the power-over tether question some time within the next year, so we're always curious to see what various builders are doing along those lines, and what kind of results they get.
If you're going to experiment with power-over-tether, please pay particular attention to safety, especially if you're going to be running in salt water with divers around.