Server-side callback to prevent connection from being established

[Torgen]

Currently Meteor.onConnection can be used to respond after a connection is established, but what if I want to prevent a connection from being established? Like suppose I want to be able to apply soft or hard limits on the total number of connections to an instance. I would like to return a 503 error to the attempt to establish a websocket, instead of perhaps establishing the connection, then immediately terminating it, because that would let the load balancer retry the request immediately, instead of making the client attempt to re-establish the connection.