I can't seem to find a way to specify the metadata for a catalyst item...

I've tried a few:
"catalyst": "appliedenergistics2:material:23"
"catalyst": { "id": "appliedenergistics2:material", "meta": 23 }

Oh, but digging in the code I found that there's a key called "catalyst-meta" 👍

Jupp, sorry, I should document this better.