I can answer my own question! If you bought Deckadance when Image Line owned it (ie before 2015, when Gibson/Stanton/Cakewalk acquired Deckadance), you can log in to your Image Line account and *carefully* read all the instructions on the My Licenses page.
Underneath the instructions about Deckadance v2.43, there is an option to download a .reg file with the licence you purchased, and even if you're on the Mac you should do this. On Windows, you just need to double-click the .reg file to install the data to the Windows Registry. But on the Mac, you need to save the .reg file in the ~/Library/Preferences/Image-Line/ folder (where ~ means your home/user folder.) That folder probably doesn't exist, so you'll need to create it yourself. But once you put your .reg file there, you're up & running.
It seems to work even with the newer Stanton/Gibson versions, I've got it working with v2.72 on macOS 10.13 now.
Hope that helps someone else!