AFAIK, the only modes available in text mode are standard VESA resolutions and timings. Thus, there is no way to overscan or underscan. For example, my HTPC is attached to Mitsubishi HDTV via VGA connection. This HDTV is capable of accepting 480p and 1080i signals, but it overscans so that the edges are off-screen. (No way to adjust this
).
In my case, the only way to get text mode to display at all was to use 640x480 mode (mode 769). This results in a 480p display, at 60hz. Unfortunately, the edges aren't visible so I can't read what I'm typing on the bottom line and I can't read the very left of the text. This is just barely usable in case I need to do some maintenance in single user mode...but otherwise, I'll ssh in from another box to repair X or anything else.
I use Xorg with customized modelines to produce a window-boxed display which fits within the visible area of the TV screen.
