There were two techniques suggested on the mailing list, but only one worked for me in Softimage 2012. There seems to be a problem with Get Set Sum (see the screenshot below). Using Filter and Get Set Size works just fine.
I was going to run the performance timers to see if there was any difference between the two techniques, but since the Get Set Sum doesn’t return the right number, there was no point.
BTW, when you’re using the state machine with the Simulation Root, anything you plug into the Execute node is processed only in the initial state. If you want to something always processed (like the Set Data above), plug it into the post-sim execute port.