I suspect you have to add a module which does the full-screen display (use CBROM to add it in).
It should be fairly trivial (implementation is left as an exercise for the student) to display a screen, wait n seconds, then progress into the POST screen as normal.
Slightly fancier is to display the screen and then grab Int 10h to absorb the screen writes (assuming the POST uses the old Int 10H routines) and detect when to stop and display the config summary by looking for the block graphic character used for the top left corner of the config summary box; also handle keyboard input to look for a keypress to show the POST screen, or relinquish and go into CMOS set-up.... I'm surprised someone hasn't already done a general-purpose one on the web, so maybe I'm talking through my fundament.
I'm not sure it's that simple. I remember eons ago reading on the Award website that full-screen logo BIOSs were available but that the BIOS needed to be specially modified. To what extent, they didn't say, but it would seem that if all that was needed was to add in a module, every manufacturer could just do it themselves.