Hi Wayne, Doug:
Powering the ROV over the tether is certainly on the roadmap for OpenROV. For the last couple of months the folks at the HQ have been preoccupied with getting the latest wave of kits out the door, and updating the documentation. Once the documentation process is complete, you'll start to see a wave of new developments, and dealing with power is one of those. The switch to Lithium cells has taken some of the heat off of this issue for now.
The MediaLink adapters are fully compatible with sending power over the tether, since they don't really care about what DC voltage is on the line. But it's not as simple as just hooking power wires up to the tether- you could do something close to this if you had a short tether with fat wires, but the current tether arrangement isn't going to be able to handle the surge current that the ROV draws at times. So what you really need is a charge controller working in concert with the ROV battery pack (sort of like the capacitor bank that Doug describes above), and pulls a fairly constant amount of power through the tether. When the ROV is relatively idle the batteries will charge, and when you're driving aggressively the batteries will discharge since the tether will not be able to provide the full amount of power needed by the vehicle.
As for Doug's question about current draw, there's an ROV at HQ now that has a current monitoring chip in the battery power lead, but I don't think anything's been done yet about wiring it back to an A/D on the Cape. Maybe this week, maybe not. I'm sure those guys wake up every day and have to choose between which of twenty things need to get worked on. If anyone at home has made current measurements on the entire vehicle (as opposed to the current monitor that measures just the Cape power), please share that with us.