FYI, it''s been a long road, but the last guest-side piece
(aka frontswap) necessary for Xen transcendent memory
("tmem") support was merged into Linux 3.5-rc1 by Linus
this week.
So it should soon be coming to a distro near you. If
you should want to play with it today, Oracle Linux
with UEK2 supports tmem and is downloadable today (free).
Interestingly, an RFC was also just posted on lkml for
tmem support for KVM.