Would it be remotely possible to designate one simple script, on one central server, to be the arbiter?   A process would, let us say, open a socket connection to this script, wait to read one byte from it, do its work and then close the socket.   This one script could be an efficient traffic cop for the whole procedure since it would be in the position to know everything that is going on.   I can’t quote an implementation off the top of my head, but I am sure there must be such a thing.

Or maybe (and I say this in all seriousness, BrowserUK could whip one up in about three minutes ...