Yeah, with a couple of dozen decoy salvos per ten seconds, a couple of dozen volleys in the train should ensure that he won't have time to kill the real volley when it arrives... assuming that he shoots at the closer ones first. Worth a try.
I make it as 22.222... volleys of 25 each, so 23 plus enough to cover for launch time inaccuracy would do the job. This cover has to be both sides of the big launch - you don't want them arriving before the 23rd, or after the final.
If the enemy doesn't fire at closest target first, the only way to get a hit with big missiles is to empty his magazines anyway. It would be interesting to see how the AI prioritises incoming - as a person I'd be prioritising closest first, but for any particularly scary targets I'd make sure to start firing at them in time to get them all.