About not seeing the GNU Mach console because the BIOS is started in graphical mode...
if I understood the problem.
Reading Grub doc...
[This paragraph probably don't work because it says it inform Linux to use text mode]
Maybe before grub-mkconfig: GRUB_GFXPAYLOAD_LINUX
or using e to edit boot configuration and add:
they say it set the video mode in which Linux is started... but maybe it would apply to Mach too.
GRUB_TERMINAL_OUTPUT=vga_text
before grub-mkconfig... I realize that if you are using a CD image... you don't easily control the grub-mkconfig part.