Links to Other Projects are taken from Wikidata and shown in the left panel. In Wikivoyage, this duplicates the Related Sites field, which is displayed in the left panel too. Example:
https://en.wikivoyage.org/wiki/Tuscany

Related Sites are created using old-style interwikis like:

[[Wikipedia:Territorial Abbey of Monte Oliveto Maggiore]]
[[Commons:Category:Abbazia di Monte Oliveto Maggiore]]

sitting somewhere in the text of the page. We would like to unify these two mechanisms, Related Sites and Other Projects, and switch to the Other Projects panel, but first we need functionality to override or at least augment the links produced by Wikidata. For example, here at https://en.wikivoyage.org/wiki/Chiusure English Wikipedia does not have an article for the city of Chiusure. Therefore, the Other Projects panel is not displayed on that page. Nevertheless, it perfectly makes sense to add a link to another article on English Wikipedia, and we do this using old-style interwiki links that are shown in the Related Sites panel.

So the question is: how can one use standard interwiki commands for adding links to the Other Projects panel?