Using Home Assistant with the Qolsys Gateway integration would give you more granular control if Alarm.com’s automation rules aren’t enough.
Alternatively, doing things like this is why I’m working on IQ Mate, a lighter weight tool than Home Assistant that just helps you do more advanced automations & integrations with Qolsys & Alarm.com, without being a full system on it’s own.
With IQ Mate you could trigger flows based on motion sensor activity and then trigger an Alarm.com event-triggered rule to control the lights. It would require an additional alarm sensor to trigger the Alarm.com rule.
I’m currently experimenting with adding a Z-Wave USB stick to it to find out what’s possible. If I can get it act as a scene controller (not sure yet if that’s possible) then you could run scenes directly from it without an Alarm.com event-triggered rule.