Any way to use UDM geofencing to trigger an iBeacon (e.g., to trigger presence in a car)?

Not directly,  but if you have an application with can call a url you can use the Portal Geofencing Node Server.

The following link is how UDM can work with the Portal Geofencing Node Server. The NS can also be triggered with a portal URL, which is how 3rd party Geofencing works with eisy.  https://wiki.universal-devices.com/UD_Mobile#Occupancy_v2.0_Node_Server

I have played with iBeacons in the past as presence detection but it was too unreliable to triangulate using signal strength, but it may work if only requiring detection instead of triangulation.   

Thanks, will give it a try.