If downtime is that critical, you should be updating during a low-traffic or overnight maintenance window when it won't matter quite so much.
A real hacky way to do this would be to install the shellcmd package and then add an earlyshellcmd to run
/bin/rm /conf/needs_package_sync
Which would remove the file that triggers the pkg reinstall.
If something breaks in a package because of the upgrade though, you've only got yourself to blame. We reinstall them for a reason, as cmb mentioned.