Good we're on the same track!!
From what I remember Lydia presenting at some point was that the transition to Wikibase on Commons would happen gradually, with more and more of the properties being displayed from Wikidata. For paintings in the Sum of All Paintings project this could already start happening, since their data is in Wikidata. Just like making infoboxes in Wikipedias from Wikidata data.
Last time I checked there were no Wikidata related Lua scripts in Wikimedia Commons (correct me if I am wrong) nor was arbitrary access available.
How I would like to start is to add:
{{Artwork
| wikidata = Q....
| artist = P...
| date = P...
| etc
}}
Since all of them are already related to the Wikidata item, we could make a preformatted ArtworkWD template where you only need to add the Qid and values that cannot be stored in Wikidata or those that need to override Wikidata values. Now we have measures, so all painting sizes can be read from Wikidata. These are the only ones that come to my mind.
- description
- geographic area
- partnership declaration
- categories
As you mention, looking at the file in Commons you would want to
- add it to Wikidata
- edit the data in Wikidata
I would love to work or let someone work on this!
Susanna