Much to my surprise, I managed to discover that Irix really does have zoneinfo support! Its just not very well documented.

First, use zic on a source file in /usr/lib/locale/TZ to build your zone's zoneinfo file.

kaji# cd /usr/lib/local/TZ
kaji# zic australasia

Next, edit your /etc/ZONEINFO so TZ is set to the appropriate path under the /usr/lib/locale/TZ/ directory, prefixed with a colon (:).

Ta-dah! Zoneinfo on irix.