Thanks for the info. I tried this, but now the LCD is black for the entire boot. No images at all.
So, I tried to leave the defaultdisplay to dvi but used the new dvimode value. With that, I got a screen with 'Overo' on it and a progress bar. It flashes, but at least it looks like the screen is being used properly.
There is an error in the boot process that may cause this issue. It does finally convert into a text screen with Angstrom image.
So, I suggest that using just
setenv dvimode 480x272MR-16@60
might be sufficient.