VirtualBox/Windows 10 tech preview get okay resolution without guest additions.

I ran

VBoxManage setextradata tv-00002 CustomVideoMode1 1920x1080x32

rebooted - and was able to select the 1920x1080 resolution.

It added:

      <ExtraDataItem name="CustomVideoMode1" value="1920x1080x32"/>

to the vbox file for the virtual machine.