I found an easy way to dual boot Batocera with Windows 10. It could be seen as quick and dirty, but I did it for my dedicated emulation machine so that doesn't matter to me. The idea is to just install grub2 separately and chainload Batocera's and Window 10's bootloaders. In my case, I have each OS on separate drives. I already had Batocera installed on one and added another to install Windows onto. After installing Windows, sure enough, it supersedes Batocera's bootloader.
In Windows,
1.) Open cmd prompt as admin and run "mountvol X: /s" to mount the EFI partition.
2.) Go here and use their tool to install grub2 to the EFI partition (the X: directory).
3.) Download my grub config here.
4.) Download Explorer++ and run it as admin then copy the grub cfg to X:\grub.
5.) Go back to the cmd prompt with admin rights and run "bcdedit /set {bootmgr} path /EFI/grub/grubx64.efi".
On the next reboot it'll show the grub2 loader with entries for Batocera and Windows.
