I've actually tried this (using a custom iBeacon advertisement data based on this article), but sad to say it didn't work. I've come to the conclusion that iOS seems to somehow know what an iBeacon advertisement looks like even when not using a CLBeaconRegion
object and it won't allow it to run in the background even as a peripheral using Core Bluetooth.
EDIT:
To add more regarding your attempt to discover Services and Characteristics on your Estimote, perhaps have a look at this answer. Long story short, if you use an app like LightBlue to discover the services on an Estimote, the ServiceUUID and the ProximityUUID are completely different. Estimote seems to use GATT for Connectivity Mode and iBeacons for Advertising mode. You can read more about Estimote's different modes here.