Hey @damienburke,
Dgraph currently doesn’t support facet variables in the upsert mutation. It is an useful functionality to have and we have had this RFC in past and hope to get it in dgraph soon.
Ref: Improve Facets mutation handling · Issue #1996 · dgraph-io/dgraph · GitHub
Currently, if you are using some client, then I would suggest you to fetch the facets by a query and then form the new facets (including the “updated” field) in the client side itself. Then you can mutate for the edge to update the facets.