Thanks for your replies. It's exciting to have my questions received so well.
I'm not quite allergic to opening the case of my unit, but I should be totally honest about my plan to be a 'consumer' rather than a 'producer' of coreboot knowledge. Investing time and money in hardware to program the flash ROM and to detect small steps toward success would take me off-line from other projects (like earning a living) that I can't afford to quit.
My little Evo has a socket for additional flash memory, but the socket is empty and that's OK with me:
I am using the T20 as a thin client for several hours each day, with gentoo linux kernel 2.6.24. I get X graphics at 1280x1024@16bpp, the maximum supported by the GX1, by using a patched version of GRUB that lies to the kernel about what memory should be used where. This patch, kindly supplied by others, sacrifices the framebuffer console displays, though. (All of them!) I am hoping that coreboot might get me everything: best-possible video, hi-res framebuffer, and efficient [non-emulated] sound support. (Then I would drop X and go with directfb for VNC.) Success stories and documentation are on the wiki at http://open-evot20.sourceforge.net/wiki/
Thanks again, ... Philip