Breakaway Glass Triggering
Various possibilities come off the top of my head:

As you said, a transparent conductive trace on the glass, then check for open.  This is certainly easy to detect, but doesn't sound easy to implement on the glass.  There are conductive and transparent substances, like titanium-tin oxide, but I don't think these come in anything like a paint you can apply yourself.  If you did, the conductive strip would probably be noticable.

IR beam just behind the glass.  When the glass is broken by the flying brick something is going to move backwards and interrupt the beam.  Even if it's not the brick but a few shards of flying glass it should still be quite detectable.  I'm envisioning the beam being only a few mm behind the glass.  I think this is the best bet as it's easy to install, should be reliable, and doesn't require additional setup each new pane.

Small piezo microphone stuck to the corner of the glass somewhere.  The breaking glass will make strong high frequency sounds that normal activity should not.  This might take a little experimenting to get just right, but should be doable.  My guess is that rejecting low frequencies will be important.  Trigger off of high volume high frequency.

Load cell weighing the window.  After the glass breaks, the setup should reliably weigh less.  Depeding on your mechanical setup, you can weigh just the glass in the frame, or possibly the whole frame.  A load cell can provide a analog signal that is a function of force (weight in this case).

Load cell on the floor behind the glass.  The weight of the fallen shards of glass should be detectable easily enough.  Possibly just looking for sudden changes is good enough.

Like #2, but refecting at a shallow angle off the glass instead of passing just behind it.

Ultrasound pings accross the glass.  The intact glass will conduct ultrasound nicely, with the air gap after it breaks much less.  The propagation time will also be longer, but I doubt you'd need to bother detecting that.

Ultrasound sonar far enough behind the glass to be out of the way.  The flat intact glass pane will reflect the sound well.  A broken pane a lot less.



While it's easy to dream up Rube Goldberg approaches to this problem, many are going to be fiddly to set up.  I would go with #2 (interrupted light beam just behind the glass).  I think that is easy to set up and should be quite reliable.