Bluetooth device identification & information during pairing

Yeah, it's entirely possible. I commented on this previously and have since looked into it further. You can have the triggering Bluetooth device send the activation signal through its friendly name.

The friendly name is the human-readable name that appears in the the list when you search for a Bluetooth device to pair with. The speed at which this activates depends on how quickly the receiving device polls for friendly names. You haven't mentioned what type of receiving device you're using but I found this PDF (Pg.35) detailing the process for updating the friendly names of in-range Bluetooth devices.

You can poll for friendly names as often as you require. If your triggering device changes its friendly name to the activation code, abc123, then the receiving device will be able to see that name without pairing and activate whatever task you're attempting to perform. This also allows you to have a multitude of activation codes, I think you can get up to 248 bytes for the friendly Bluetooth name.

You may also look into intentionally opening yourself up to Bluejacking. I'm not familiar with it, but if security isn't an issue it could work.

You have 2 issues in here:

1) You want to communicate with more than 7 slaves. It's not possible for a master to have more than 7 slaves. However, it is possible for a device to be a slave in one network and be the master of another (called piconet). However, I have not seen any device that uses this (processing power would also be higher).

2) The issue of pairing the arduino automatically to a device, this is a software issue you need to solve in the app. The Android phone will detect 3 bluetooth devices. Since each has a unique identifier, you can have a simple algorithm to select one of them depending on the criterial, without user intervention.

In Bluetooth you must pair. This has to be done to keep both devices synchronized and talking. I think you're slowly going to the proprietary RF side. Perhaps you should think about your application and see whether you can use a non Bluetooth solution that can enable you to scale.