MEMDISK is part of the Syslinux project; it loads disk images into memory, sets an int 13h hook and then does a BIOS boot from the image; this can be used as part of a PXE boot environment to load installer disks, however the disks are not accessible from inside the FreeBSD kernel because it doesn't access disks through BIOS APIs.
This patch detects the disk images in the loader, and passes their address and length as a driver hint. When the md driver sees the hint, it maps the image, and presents it to the system.
It seems like FreeBSD 8.x used mfsroot on installer images, which would work with MEMDISK, but all releases since then stopped in favor of mounting the cdrom or memory stick directly. PXE booting with the installer root mounted via NFS could probably also work, but is a more involved setup than loading an image.
I'm not sure if the printf'ing here is too verbose (probably).