Yep, overscan is a pain. I have tried two TVs and both overscan on 720p resolution. Going through the TV settings in both cases reveals options to make the overscan worse, but none to correct it.
However they both work perfectly when driven with their native resolution. For one, that is 1080i, which displays beautifully. For the other, which I actually want to use with the OPi PC, it is 1360x768. Odd resolution for a TV but there are quite a few older sets like that around. And if you drive it at that resolution, whether HDMI or VGA, there is no overscan.
But will the OPi do that? Well the OpenELEC ROM includes a script.bin to set the resolution to 4k (= 3840x2160 @ 30hz) which translates to mode 28 in the FEX. So there are other modes possible than those listed in h3disp -- I'd very much like to know what they are!
You can view the missing bits of the display from another device using x11vnc. This has another advantage -- sometimes something will go wrong when playing a video, and even after exiting mpv a corrupt image will be left covering part or all of the screen, making it hard to use. However the vnc viewer shows the screen as it should be, so you can start mpv again and close it properly, which gets rid of the corruption.
Computers that always work right are boring!
--Jal