This isn't the first time there's been a service that needs to run during shutdown, after other things have been shut down. SunUC's automatic mechanisms for installing "single user" patches have roughly the same requirement, give or take exactly where during shutdown. (I have never looked at that sequence in detail, but I have seen that it runs too early in shutdown.)
Assuming that SMF's shutdown operation is to stop every service, it seems like a "last minute" shutdown service would want to be depended on by everything else, so that everything else must shut down first. (The mirror side of that is that it starts first, but since its start method would presumably be :true that doesn't seem like a problem.) However, I suspect that for this particular operation (cutting power to the UPS) a kernel hook _is_ required. I assume that when you cut power to the UPS, pretty much instantly the computer loses power. That means it better happen after the final sync, which is down inside the kernel inside uadmin(2). (Perhaps others realized this, but I don't see it clearly in the messages.) Probably a ZFS-based system can survive a power failure. A UFS-based system used to be able to, but at least the last time I checked was not able to: it often ended up with inconsistencies requiring a manual fsck.
