squashfs-tools with support for LZMA/XZ was uploaded to squeeze-backports.

4dd40ab9 · Tails developers · 7e76d0d0 · 4dd40ab9
Commit 4dd40ab9 authored Jul 29, 2011 by Tails developers
--- a/config/chroot_apt/preferences
+++ b/config/chroot_apt/preferences
@@ -23,6 +23,10 @@ Package: macchanger
 Pin: release o=Debian,a=unstable
 Pin-Priority: 999

+Package: squashfs-tools
+Pin: origin backports.debian.org
+Pin-Priority: 999
+
 Package: virtualbox-guest-dkms
 Pin: origin backports.debian.org
 Pin-Priority: 999