Hi Kevin,
It's true that we don't have an endpoint at this point to list/enumerate all the labels. If you have a specific label you'd like to look up, the `Label` option is a valid type for both the `search` and `searchSuggestions` endpoints.
Some of the objects also have a `label` value returned, which you could then use when making the `getArtistsForLabel` call. For example, Albums have the field but it's listed as an Other Field, which means you need to explicitly call for it in the `extras` list
Hope that helps, sorry for the delayed response,
Rebecca Stecker
Rdio API Engineer