I've created a patch for genkernel-3.0.2g which allows to specify which device to mount as a CD-ROM for booting LiveCDs using 'cdroot=/dev/sda' syntax. Why? My initial intention was to fix bug #22157 which is a request for Gentoo LiveUSB installation mediums (for installing on computers with no CD-ROM drive), but the new feature has proved useful for testing LiveCDs without burning a CD-ROM as well. I am currently working on a simple catalyst liveusb stage for automatically producing a LiveUSB tarball/zip file from a livecd-stage2. This is tracked in bug #22157.
Created attachment 40519 [details, diff] Patch for adding cdroot=/dev/foo feature
In genkernel 3.1.0; marking as FIXED.