First off, I'm not at all familiar with MythTV, so I don't know how easy it is to integrate any extra functions into it.
As far as mplayer goes, you can put pretty much anything you want on the mplayer screen by using a video filter. Here's an example
of what you are asking for. But it seems to need a patch to the mplayer source code, so it's not something you could easily add to your existing system.
Mplayer does have an OSD menu system driven by configuration files. It might be possible to hack it into providing the information you want, but again it's probably hard to integrate into MythTV.
I get the feeling that MythTV itself should be taking care of this, instead of users trying to interact directly with mplayer.