Have you tried SeaBIOS for your payload? It runs the option ROMS directly instead of emulating them. As an added bonus it will use your original grub installation.
I have considered this but didn't test it yet as filo also had a patch to support the dl145g3, and i'm unsure if i need to port that patch to seabios as well: http://www.coreboot.org/pipermail/coreboot/attachments/20090331/9e221f84/att...
If SeaBIOS doesn't work for the onboard graphics or the VGA card, send the logs.
Thanks, Myles