Getting the easy question out of the way first, Limo transfers aren't available on award flights (although you can add them for 10K miles per leg - but I'm sure you realise that's a poor exchange).

As for availability; two things to try. On Virgin's own website, if you've registered your Flying Club account you can book online (or at least go through the motions to see what flights are available). Also, on the VirginFlyer.com site there's a Sabre tool which won't show you the specific award code buckets; but if availability is available in 'Z' class then Virgin have been known to release that for Award flights on request (it's a call that their Revenue Management will make, but you can get in touch with them via the Flying Club team). Be as friendly as possible and you might get lucky

Pix