Alternately, are there any existing mechanisms for monitoring route announcements which can provide near real-time alerting when any prefixes within specific subnet ranges are announced?
Not that I know of. You can log into route-views.routeviews.org and use the cli to watch it, but that is a manual process.
Hope this helps,
Dave
To my knowledge, the myas-tool/-service from RIPE NCC is kind of doing what you like to achive.
MyASN is working on user-based. To get the alarm for unexpected routing patterns, you should set it up an account beforehand. I think for Kevin's situation, we have other tools. One is called, "Search by Prefix" and other one is BGPlay. Both tools are running over last 3 months routing data. URL for those tools, http://www.ris.ripe.net/cgi-bin/risprefix.cgi http://www.ris.ripe.net/bgplay/ Arife