Counting clients for all instances in an application

I have multiple instances in an application and want to count the number of connected clients in each instance, and then make this information available to all clients connected to the application, in ex. a SharedObject. This way I can sync on changes every time a client is connected or disconnected without havint to write externally to a php page, and then use a timer to get this information back to each client, this slows down the sql server when hundreds of users are connected to hundreds of instances.

Is there a equivalent function to application.clients.length for the application across all instances, or a way to make a SharedObject run "globally" on the application and not on each instance?