Currently /etc/init.d/halt provides a hook into the halt process via an
/sbin/halt.local file. If this file exists and is executable, it will be run
during the halt process. This makes it possible for system administrators to
write their own halt modifier (typically needed when dealing with UPSes and
possibly other types of packages).

I think it would be very valuable to extend this functionality of
/etc/init.d/halt to support a /etc/halt.d/ directory in addition to the
halt.local script (Why is halt.local currently not located in /etc along with
rc.local and such but in /sbin btw?). This would allow a package (an UPS
software package as an example) to drop a halt modifier script in /etc/halt.d/
to take care of prepping the system for halt (rather that poweroff) and initiate
a delayed UPS poweroff, if a mains powerloss condition was detected. Currently
the package will have to hack either /sbin/halt.local, which could be difficult,
or /etc/init.d/halt which would be just plain bad. Usually they will simply
leave this out and the system admin will have to set it up manually, for the UPS
to work properly.

The current version of the /etc/init.d/halt script (from initscripts-8.60-1)
includes UPS handling code for the NUT UPS software(i think). NUT could easily
be made to use the new /etc/halt.d/ infrastructure too, and /etc/init.d/halt
could get rid of the non-generic UPS handling code, which IMHO should be avoided
anyway...