You can put a Command Event at the very beginning of your script - "SSA:Wait for audio trigger". This pauses the script (literally, as if you pressed "P"), and waits for an audio trigger. This is effectively the same as using Audio Triggered playback, but with the important advantage that only the "P" key or an audio trigger can start the script playing. ie. alt-tab (or any other key) will not prematurely start the script. So, you put the script command at 0:00:00.00, select (non-audio-triggered) script playback, click OK, and press any key... and presto, your script will wait until you either press "P" or there is an audio trigger. The "P" keypress will be recognised even if SSA does not have focus - if you have "K/B intercept" switched on in SSA. This provides the "foolproof" way of synching the script with the movie, that you were looking for. (Well, almost. You'll still have a problem if you have to use the "P" key for something else ^_^) The command can be easily inserted, by pressing ctrl-A at the top of the script (to add a new line), then select Edit | Event Type | Command | Wait For Audio Trigger. Afterwards, check that the Start Time for the command event is 0:00:00.00
All credits goes to Kotus for these instructions. I will re-write these instructions as soon as I get round to trying these steps myself and grokked them fully :-) (Probably within a week hopefully!)