sorry to reply to myself, but I have just found[1] first moves towards sysv
ipc checkpointing in 2.6.22, so I guess it's wait and see game for me now.

 And to modify my orginal question - are there any additional limits on
checkpointing? Until now I have found that you can't use SYSV IPC and you
can't run stuff via 'vzctl exec' or 'vzctl enter'.

1. http://www.mail-archive.com/devel@openvz.org/msg02797.html
-- 
Key fingerprint = 40D0 9FFB 9939 7320 8294  05E0 BCC7 02C4 75CC 50D9
 Total Existance Failure