The way I imagine this would be to have a parser function called {{#wikidata:}} (the name is obviously just a suggestion). So for any page on the Incubator that is about Norway, you would add {{#wikidata:Q20}}. The availability of this parser function should be limited to only a small number of wikis, via a configuration switch in InitialiseSettings.php, let's just call it wgAllowWikibaseAssigning, which defaults to false, but can be set to true for the relevant wikis (maybe also only allow it for a limited set of namespaces?). For enabled wikis, the parser function should do what the description says; for all other wikis, it could either just show up in plain text (like {{SHORTDESC:}} does outside of enwiki), or just not show at all, or not show, but add a tracker category. The latter would be useful for bots to connect articles to Wikidata items after a wiki has been approved and exported from the Incubator to a proper separate wiki.

I do not suggest this approach. What I suggested (since 2017) is a "virtual site" solution: You can add a sitelink to xyzwiki, which will automatically matched to (i.e. referred to) a subpage of Wp/xyz in Incubator. This also allow creating an production wiki without any change of existing sitelinks (i.e. you can switch what a sitelink refers to in configruation and all existing sitelinks will continue to work).