You can now add metadata to Topics via API (only). Please refer to the API documentation on making API calls:

The property for the Topic metadata is... metadata. The API will expect a hash for this data:

  forum_id: 123,
  title: "My title",
  metadata: {
    product_id: 100

Making an API call with the above JSON payload will insert a metadata with a key of product_id and a value of 100.

With this, you can also query Topics via the JavaScript endpoint at /community/champ/latest.json?metadata[key1]=value

You can also chain the metadata conditions as well:

So using the above example, you can query for that particular topic like so:[product_id]=100

Please report bugs :-)