There's not much hope of that occurring. Zimbra HQ drag their heels a bit on supporting the latest versions of OSX on Intel CPU's so outdated PPC kit is way off their radar. As a company they need to fund development of the new OS/distro's that paying customers demand rather than back-fililng every OS/distro combination out there for the community - if you release something there's a cost, even if it's unsupported (you always have to support it in some way). The array of OS/distro combinations and the level of assistance that goes into the OSE is already pretty decent if you consider that.
NB: There is an officially unofficial source for Zimbra PPC builds located here
Browse Zimbra Community Builds & Projects Files on SourceForge.net but they're always a few builds behind the current (latest is 5.0.14). ISTR reading somewhere (on here possibly) that the PPC builds posted there were contributed by a Zimbra staffer who ran his own server on an early Mac mini and re-compiled them to suit his own needs once in a while, but if that's the case he clearly doesn't run upgrades on his own server too often

I've used the 5.0.14 build on an elderly 1.3GHz/1GB/40GB G4 Mac mini a couple of times to make an emergency mail server when things have conspired to make my normal one unreachable. Startups and service start/stops were dog slow but it ran fine once it got going.
Christian