Is this of any use for Logic or Garageband? I found an old version of Apple's Developer page, but not sure it's useful. Any thoughts?

 

If you want more control over how to create your own Apple Loops, then definitely yes.
It also lets you batch process/convert audio files into Apple Loops.
And also, you have to know that you can turn an audio file into an Apple Loop even if it doesn't loop. These are called "one-shot" Apple Loops. You can use this to "tag" a big collection of audio files with metadata, import them into the Loop Browser in Logic or GarageBand and have "searchable" access to them. Viola

 

I used it recently to convert all my sound bites (FX, risers, one-shots, etc etc) into apple loops to be used in Logic Pro X. Much better for me to have them sorted & available for easy preview. I haven't found a better way of categorizing all my files, so I can only recommend it.

You have to download the latest version from the apple developer page (create free account)....the interface looks like it's 8 years old, but it works beautifully to batch-convert files.

 

If it weren't, it wouldn't exist, would it?

The version I'd obtained contemporaneously with Logic Pro ("Studio") works fine on El Capitan; don't know about any more recent version.

It takes a while to learn how to set key, beat-transient markers and tempo so that loops will work at the keys and BPM intended.