Apparently pages in the Data: namespace on Commons can be categorized these days – see T387092, T387094, T387090. Would be nice to support that, I guess. (We already support the proofread-index content model, so there’s some precedent for non-wikitext content models. It would be the first non-wikitext content format, but given that it’s a much simpler format to work with – a dedicated key in JSON – I think I could live with it.)