We’re definitely interested in this feature, could use it ourselves too. Hope it gets enough votes to bump its priority.
In the short term a kind of workaround will be soon available in the form of a new Bitrise API endpoing (Bitrise.io API v0.1 (Work In Progress)) where you can fetch artifacts. Not perfect, but with that you’ll be able to store your metadata as an file/artifact attached to the build and get it through the API.