Why is the symlink provided? Where is the definition of the ups-monitor virtual package written down? What is using this symlink? I assume a good solution should take into account the answers to these questions. :)
It's there to be called by various parts of "init", though right now I think only /etc/init.d/halt calls it (to perform last-moment power-down of the UPS).
There was a long thread on debian-devel about this feature. There was no way to place it under rc.d because it had be be executed absolutely last (after filesystems had been unmounted). Not all UPS systems provide a delay between the signal and power-off.
-- Brian