Try replacing mousedown
with dataRequested
.
On mousedown, the flash object will check and see if the clipText has been set. If it hasn't, then it will fire off a dataRequested event. If the html object has data-clipboard-text or data-clipboard-target then ZeroClipboard will take care of getting the data. However if it hasn't been set, then it will be up to you to clip.setText from that method. Which will complete the loop.