There are certainly plans to continue having a VM image, it does make for an excellent quick showcase.
I did let the team know about the connection issue & will fwd your suggestion about 'pulling the VM's because they're so old, to not give a false impression if updates.zimbra isn't planned for fix soon.'
Remember the VM's are beta; just trying to give you a hand to get started (as the update URL is not my realm & I have no idea what the license terms were with rPath) by pointing out that rather than the beta you might try the stable GA. I think you'll enjoy 5.0.x as it's a huge improvement upon 4.5.x
Why we switched to Jetty » Zimbra :: Blog Getting an OS + Zimbra installed is pretty straightforward, with missing package dependencies shown early in the process (and you'll get a port conflict if you forgot to disable say sendmail) before the install really starts, it's the DNS that gets most people.
(I'm not sure how the "my only option for the proof of concept" comes into play? I assume it's not 'time involved' because I gather you'd need to get experience installing Zimbra and thoroughly vet it for your needs anyways - and you can do that all in VMware, Xen, Virtual Box, etc, etc. Sry I'm missing your point there if you care to elaborate.)
If you don't have a DNS server handy here's some examples:
Split DNS - Zimbra :: Wiki Making Zimbra & BIND Work Together » Zimbra :: Blog
There's a bunch of open RFE's on VM's:
Bug 14927 – rPath Network VMware Beta & even
Bug 13508 – Live CD
(The community could probably even maintain some current FOSS images.)
Single Server Installation Guide - Network Edition