Ah, no problem!
So there is nothing related to grub at all in pmOS_boot...
However the system is boot-able through UEFI entries! I don't know why I didn't try that earlier.
My workstation's BIOS is all fancy and let's you add custom entries, but it was easier to use MX's UEFI manager. I simply added a new entry to pmOS's BOOTX64.EFI.
I still would like to boot it through my GRUB.
Also thanks for the piece of info and all the interest in helping me so far :)