I worked at this some more and managed to make an iso that will boot to an Xfbdev gui in both legacy-bios and uefi boot from a CD.
On my hardware at least, this will use the native screen resolution rather than being limited to something less. Depending on how your cd/dvd is connected, even with waitusb, you may have to let things boot to the gui (it takes a while) and then exit back to get a console at native resolution.
Now the only problem is that the iso is 50% larger than the standard one as I cannot (yet) get grub to use only the required modules in either embedded or normal mode.
See:
http://tinycorelinux.net/6.x/x86_64/release_candidates/TinyCorePure64_mb-6.1rc2.iso