I don't know much about this stuff, but isn't there a way to fork something as a
background process? Something equivalent to me typing "./my_failover_script &" ?
So assuming nginx could simply make a shell system call or somehow make the call
but tell it to run in the background, then I imagine the nginx thread that would
call out to run the script would return almost immediately. At any rate, even
if it took a little time, this should be a rare event, like maybe once every few
weeks if things are running smoothly.

And if it can't be launched as a background process, I would simply make the
script it calls do something like flag another process to run, and then exit
immediately so the thread returned quickly. The main thing is that I just need
nginx to signal me somehow when it takes an upstream server out of rotation.
Doing this by sniffing the log file is a hack at best.

Yes I had a look at the tutorial on building modules. Unfortunately it is over
my head. This is why I am hopeful that the developers will recognize the
importance of such a "hook" to an external monitoring system would be very
useful especially if it is straight-forward to do. Thank you.