Sergey, I am not sure I understand why having 32 bit Zimbra on Solaris can not use ZFS. Solaris can support 32 bit application natively even when it is running 64 bit version of the OS. AFAIK, most Linux 64 bit distros expect all their apps to be 64 bit only.
If there is a demand for 64 bit Zimbra, then I will make an attempt for the same. I am in middle of another project. Once that is done, I plan to setup nightly build system for Solaris (sparc & x86). Let me see how it progresses.