I signed up this morning for ttpalerts.com, which is $12 for a month of alerts and doesn't seem to limit the number of locations you can monitor. I initially put in the NYC-area locations (JFK, EWR and Bowling Green) plus AUS and ORD and the Chicago Field Office as we'll likely be visiting AUS and CHI before our international trip. It seems to send an alert whenever the first-available appointment at a location changes. So far today Chicago seems to have opened a whole bunch of appointments in late October and a smattering in November and December, and AUS opened almost all of December (I'm pretty sure both those locations were showing "no appointments available" when I checked this morning). I also got an alert about one at EWR this afternoon (someone must have canceled).

So it does seem to work as I'm definitely getting notified about things that weren't there when I looked earlier today.

But there are a couple caveats:

1. There's a ton of noise (I've probably gotten 20+ alerts already today in the couple hours since I signed up). It seems that whenever somebody grabs the first-available appointment at a location, I get a new alert about the new "first-available" appointment at that location. It would be nice if I could specify date ranges or something so I wasn't getting alerts about Chicago for October when I won't be there until late December. I guess I might be able to do some email filtering at my end so that it auto-deletes most of the emails that aren't relevant to me.... But because it only tells you "first available" you also don't know if someone just canceled a single appointment or if the location just opened up a new week or month of appointments for booking.
2. Appointments get grabbed very quickly and I'm not sure how often it scans for the new appointments. While poking around on the actual "reschedule" page on the government website, for example, I noticed an appointment popped up for JFK tomorrow that went away a minute or two later, and I never got an email alert for that one.