I started discussions with AppStream devs to add new tags in the AppStream file. This file contains upstream metadata and it is cross distro in contrary to our d/upstream/metadata.
If you are using the d/u/metadata to store bibliography references or registry refences you can take part of discussions here [1-2].
Currently, they agree to add registry references but still need to find the tag name. Concerning the bibliography references there is no consensus but I still think from a scientific point of view it could be useful to have a bibliography in this file and in the software center.
The idea is to move some information (or all?) from debian/upstream/metadata to the AppStream which is cross distrib and ideally maintained by upstream. Ok we already have some troubles to get manpages from upstream so an AppStream file is a dream :-). But at least if upstream wants to provide metadata, there is a possibility with this file.
So, if you are interested (or not :-), you can go there [1-2] to give your point of view.