I finally have this working how I want it to.
I'm one of those people that prefer 4:3 content stretched to fill a 16:9 screen. By default, any mythtv recording I playback from the datagrid would play back in 4:3 and not fill the entire screen. I would end up having to change the aspect via the UI2 menu for each thing I watched - which got old.
I finally started playing around with recording profiles in MythTV, and with some experimentation I found that mythtv will produce a 16:9 .mpg file from a 4:3 source. All I had to do was enter the setup for the frontend where my capture card was located (the core) by using the TV scenario, and hitting escape while LiveTV was playing. From there I went to Setup->Setup/Utilities->TV Settings->Recording Profiles. Then in the Default profile, I changed the capture resolution to 720x400, and the aspect ratio to 16:9. Now my recordings come back already at a 16:9 ratio so they playback as I want them to in both MythTV and MPlayer, and every other media player I could find.