I can only help so much, but I been messing with mdconfig
for mounting images.
You really ought check it out. mdconfig
the image and mount the installer.
I asked one time here about images and mdconfig "can you grow an image" and the answer was yes. No explanation.
So then I had no real need but now I see how you do it.
When you create an memdisk{md0} for a disk image --usually you just let mdconfig size the memdisk to the image size. But with mdconfig you have a size option where you make the mdconfig memdisk size bigger than the image you put on the md0, then the image has somewhere to grow,
Just make sure to exit properly with a umount
and mdconfig -d -u 0
Why grub? I am presuming this is for your laptop 32bit conversion???