Just continuing to answer my own questions here. It looks like the <dns>system_services entry is likely related to a pull requests here https://github.com/pfsense/pfsense/pull/679/commits/513d61a8934989be0b9332d29d0f80926302344e#diff-163d9a46abf34586e1f87297c7670978R193 that was never actually merged.
So basically it does nothing at this time.
But it would be really great if that function would exist as mentioned in the pull request. Instead of hard coding a check for specifically unbound or dnsmasq if any package that provides the DNS system service is enabled then those triggers would apply.
Since pfSense is open source I may send a new pull request myself at some point when I'm done my current port.
What I've done in my package is I wrote some code that configures unbound on a non-standard port and makes it enabled while making sure dnsmasq is disabled. Just so that any pfSense functions that depend on the DNS Resolver/Forwarder (which we've replaced) being enabled will work.</dns>