It took some time to find a usb stick that would boot in legacy-bios mode and then only corepure64, but anyway..
If I boot in legacy-bios mode without a "vga=" code, Xfbdev gives a /dev/fb0 error and will not start.
If I boot in uefi mode without a "vga=" code, Xfbdev starts at the native resolution of 1366x768 (which has been set by grub2)
I tried booting in legacy-bios mode with "vga=792" and got the grey screen with an "X" cursor - on checking, I found .xsession was trying to use Xvesa, even though /etc/sysconfig/Xserver=Xfbdev and Xvesa does not exist on corepure64.
On correcting .xsession to use Xfbdev, startx resulted in a normal desktop at 1024x768
I tried to use "vga=895" (0x017f+0x0200=0x037f=895), but I got an error about this being depreciated.
For reference: $ dmesg | grep fb [legacy-bios vga=792]
...
vesafb: mode is 1024x768x32, linelength=4096, pages=0
vesafb: scrolling: redraw
vesafb: Truecolor: size=8:8:8:8, shift=24:16:8:0
vesafb: framebuffer at 0xe0000000, mapped to 0xffffc90000400000, using 3072k, total 3072k
fb0: VESA VGA frame buffer device
$ dmesg | grep fb [uefi gfxmode=1366x768x32]
...
efifb: probing for efifb
efifb: framebuffer at 0xe0000000, using 4160k, total 4160k
efifb: mode is 1366x768x32, linelength=5504, pages=1
efifb: scrolling: redraw
efifb: Truecolor: size=8:8:8:8, shift=24:16:8:0
fb0: EFI VGA frame buffer device