A given NVRAM entry points to a particular distro shim+GRUB, and it's not possible to use either chainloader or configfile to forward to another distro's GRUB. If GRUB wants to support the multiboot Linux case, it needs some way of discovering other distro's shim+GRUB comb in NVRAM and on the EFI system volume, to populate the GRUB menu. And then the user can select that distro in the GRUB menu, which causes GRUB to use --bootnext to do a one-time boot using that UEFI boot entry, thus loading that distro's signed shim+GRUB combo.
Or, is this just not a priority for the foreseeable future, and users will have to use the UEFI firmware's built-in boot manager for this use case?
Thanks,
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Chris Murphy