You can run this directly from within rc.local(8). If /etc/rc.local exists, it is executed with /bin/sh just before starting local daemons. See the last lines of /etc/rc for details.
I use daily(8) with an /etc/daily.local script on my servers, and use the -c option because I still execute syspatch manually. I may need to restart daemons for userland patches or, more freqently lately, I've needed to reboot after a kernel update. And those I want to do in specific server sequence to avoid any operational outages.
Code:
#!/bin/sh
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/usr/sbin/syspatch -c
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