pfSense

Not sure if this is possible under the current system...
If it is possible to determine what version of [a] package[s] will likely be required post-reboot to complete the upgrade, would it be possible to download them and stage them on disk before rebooting? On a system with only two packages loaded, with a decent internet connection, it still takes ~5min to complete that part of the upgrade!
Coupled with issue #3013, this can make upgrading - even in a CARP environment - a painfully long outage. Pre-staging the packages would at least avoid the unexpected (if the upgrader is tired and hasn't done this for a while) delay in the middle of the upgrade.
Not quite an in-service upgrade a la Cisco or Juniper, but on a firewall with lots of installed packages, this could easily shave 45min off the total user-visible downtime.
Note that although core routing functionality is up and running quickly, some packages are critical to normal operation of the firewall, e.g. bgpd/ospfd, squid, etc.