Hi - I'm also very interested to hear the outcome of your tests with stream expressions, but just a thought to help you sleep for now, as I totally understand.
I'm in the same position as you with out-of-hours alerts, and its important to react to things you can deal with and not things that you cant. After all we have day jobs too right!:
Many of these 3rd party external monitoring companies offer you the ability to omit certain URLs from the test/monitor. Not sure who you use, but it may be worth checking with their support staff if this can be achieved. You could always have a seperate monitor that just checks the 3rd party URLs and alert a seperate list of people, as you would still want to know I'm sure.
Of course apologies if you have already checked with them, but we've had to do this for several 3rd party URLs that may go down outside our control.
Sam.