Right now, I'm doing most of my testing using a qemu/kvm guest and
generating the initramfs on another box.  I then can boot the guest
using qemu's -kernel and -initrd options.  Currently supported rootfs
types are regular partitions, root-on-lvm and root-on-lvm-on-encrypted-pv.

generate.sh exists and will build an image.  Right now, it depends on
some existing pieces of the Fedora initrd infrastructure, but moving
those out is a priority now that the basic functionality has been
proven.

To use, just run generate.sh with an output file for the initrd.  It
will copy over some binaries as well as the appropriate shared library
dependencies.  If you are doing a rootfs type that requires modules
(on Fedora, this is just encrypted root now), you'll want to have the
modules tree to copy in in a modules/ sub directory.