I have an app with a privileged helper tool which needs full disk access in 10.14 Mojave.

The privileged helper tool is (as you'd expect) a binary rather than an app bundle, so it's not showing up when I press the plus (+) button in Security & Privacy -> Privacy -> Full Disk Access. Neither can I drag and drop it in there.

I've looked around online and I can't find the answer to this anywhere. I've found a couple of threads in Apple's Developer Forums (most useful being https://forums.developer.apple.com/thread/107546 ) but I still can't get it to work.

I see that it must be possible because Carbon Copy Cloner is able to do it - you can add their helper tool either by using the plus button or drag and dropping the binary.

My helper tool is signed properly, has a bundle identifier and otherwise works fine...it just isn't permitted access to the full disk.