I've just had a thought.
Apparently, the mac unzip doesn't like spaces in the file name. Now the Heaven's Grave file name doesn't have any spaces, but it does have an apostrophe which some systems don't like in file names. It's also the only one of my game demos to have an apostrophe (and, Desrin, you mentioned all the other BC&T demos you tried had worked), so I thought it was worth a try and changed it.
Could someone for whom it was not working before, give it a try now, please?