Boot off PCIe NVMe SSD by chain booting into its grub

[Vladilen Kozin]

Hello grub users.

Not the first to bring this issue up, but I am looking for a rather
particular solution. Problem is that mine is an old system with a
bunch of spinners and USB-2. I installed PCIe NVMe adaptor and now
have a fast SSD. Sadly, old BIOSes can't see those for purpose of
booting. I can install an OS (Guix in my case) onto that SSD without a
problem. To boot into it, you'd want indirection i.e. kick off e.g. a
USB drive and then end up on that SSD. I've successfully been doing
just that while running FreeBSD. From similar questions on this list,
far as I could derive the solution was something like this:
- install your OS on that SSD,
- put /boot onto USB stick
- copy over whatever incantation /dev/SSD/boot/grub/grub.cfg has to
load the kernel and initrd,
- but also copy your kernel ash initrd image over to /boot/ on that
USB and have Grub boot into that.

At least this is how I understood what had been suggested in the past.
I've not seen actual steps described in any detail.

Even if I figure how to make the above steps work my problem is this
"copy kernel and initrd image" over to the bootable partition, since
the whole value proposition of Guix is that you can build your system
at will and newly build systems are added to Grub menu and you can
boot those and roll back etc.

So, instead I would like to also employ indirection but somehow chain
into Grub installed on that SSD. That's probably not Grub's own
"chain" cause it still won't see that drive. Maybe I could put some
kernel / initrd on USB that would boot with those drives visible and
then have it run Grub on SSD somehow? Or maybe I can load a kernel and
initrd into memory which will probe and find that drive and then Grub
would be able to see somehow?

Or maybe there's some magical Grub module already that can find these
PCIe NVMe SSDs?

Thank you
-- 
Best regards
Vlad Kozin