Do you even need a VGA BIOS in an emulator? It's an emulator - just write to segment B800 or wherever and have the host-side process just watch that page of memory.
Again, qemu works fine without a retrocomputing compiler. Is VirtualBox doing something that prevents it from taking whatever approach qemu uses? (qemu even has the cool thing where "-curses" will cause it to copy segment B800 as text to your host-side terminal emulator and avoid rendering graphics at all.)
