The only work-around I know is to set up a Scheduled playlist for each occurance of the event during the day, and set each one to play daily (so for one sponsor message on the hour+half hour, that's 48 playlists).
Somebody reported one snag with this method is that if you do a live show for, say, 4 hours, after the live show has finished the Auto DJ tries to catch up by playing all the missed items (e.g. you'd get the news 5 times).
The facility you describe has been requested many times before, but Centova don't seem to geddit how vital it is for some of us to be able to run news/ads/featues at specific times - the limit of once per day is nonsense - it needs to be at least once per hour/2/3hours etc (if you can't predict when an item will play, you can't promote it).
Including the feature you mention for Interval playlist to simply start at specified times would transform what I could do with my stations.
A further improvment would be the ability to set day/date rules, so, for example, the ad would play only during the shop's open hours, and the news could be set to play only Mon-Fri daytimes etc...
I'm sure this, and more, would be totally easy for the developer to implement - computers excel this sort of thing!
Cheers.