I’ve gone with area:highway=forecourt in https://www.openstreetmap.org/changeset/162887146, which I made up from scratch. I couldn’t find any examples of other people specifically tagging these areas, which is a bit odd. The best I found are various versions of service=forecourt, but those are only used on petrol station forecourt ways (https://overpass-turbo.eu/s/1ZiF).