To help custom data authors we should recommend particular file suffixes/extensions (e.g. .html-data.json) . When using these extensions they then will automatically get the schema association with the proper schema..

To support that. we add to package.json of html-language-features:

{
	"fileMatch": "*.html-data.json",
	"url": "https://raw.githubusercontent.com/Microsoft/vscode-html-languageservice/master/docs/customData.schema.json"
}

Note, it's what we do for color-themes already:
https://github.com/Microsoft/vscode/blob/555727f1c7c82d55503ff15021a83ade8ef858a0/extensions/extension-editing/package.json#L37