> How about instead defining a 'startup' program that starts jobs in their own cgroup, notifying interested programs through some mechanism (init being one of them, but probably not the only one) and then get CRON modified to use this program to fire off the apps.
Internally "cron" subsystem is implemented by a separate utility. It just uses the common systemd infrastructure and can be easily developed separately.
It's certainly possible to hook up systemd to launch services using DBUS interface. And you can then use it with any kind of triggers - I've actually used good old cron to start systemd tasks for a while.