It looks like being able to use lzma (actually xz) is going to be possible for this release. I have a concern about making the versions of livecd-creator squashfs-tools that supports this available on whatever system is used to build the official spins. Currently the support for this is going to be just for F15. squashfs-tools 4.2 is needed for the proper support. I'd rather not do that update on F14 or F13.
Also xz compression doesn't get made the default for livecd-creator in F15 or the F14 version of livecd-creator is used (with an updated squashfs-tools), the compression type will need to be specified when doing the build. Which is different than what has been done in the past.