UltimateBootUSB for FreeBSD

I'm trying to create an 'UltimateBootUSB' for FreeBSD, which boot various versions of FreeBSD, Linux and Windows. If I have various boot images on small USB sticks, how would I best copy them to partitions or folders on my big USB stick so that I can boot them via Grub?

I've already managed to get most of these Linux distros configured


I'm also trying these,


although I can't get ArchLinux to work
Looking at various examples of grub menu entries which run on Linux, I see a number which use MEMDISK. Is MEMDISK available as a FreeBSD PKG?