I have found the answer to this thread. The firmware of this T1000 was on a relatively old revision (6.3.12) and upgrading the firmware to the latest available revision (6.7.13) resolved this issue. I can now use LDOM in OpenBSD 6.3 as the man pages describe.
As noted in the thread "Sparc64 LDOM not working past OpenBSD 6.5", I cannot configure LDOM with OpenBSD 6.7 or later (I haven't tried 6.5 or 6.6 yet). Thankfully, domains configured on OpenBSD 6.3 work properly with a OpenBSD 6.9 host system.
