Does your servlet architecture allow you to listen for connections on the java side and maintain stateful socket connections, or is it HTTP GET/POST only? Although FMS can't listen for connections other than RTMP, it can initiate a stateful connection with another service.
It seems to me that the optimal solution would be to have the FMS application open a socket connection to the servlet onAppStart (see the XMLSocket class in the server side actionscript docs). If the servlet doesn't support it, it might make better sense to refacor the servlet. Otherwise, you may be stuck with something like this:
1. Servlet invokes broadcastMsg on the admin service
2. The servlet gets a nondescriptive success/error result.
3. Admin service dispatches the message to the FMS application
4. The FMS application updates the shared object (you'd handle this in your server side AS).
5. somewhere in the data the serlet sent, there will be data the FMS app needs to send a message back to the servlet (url, whatever)
6. The FMS app makes an http request of the servlet, and sends whatever data you need sent.
That's a lot of processing for updating some values in a shared object, and since you mentioned there will be a lot of these events happening, it might be a strain on the admin system (and might pose serious challenges if/when you need to scale the application).