Indeed! I believe that feelies are not merely extras but core elements of a game’s text. This is a ground-level assumption in all of my Infocom writing.
I do wish feelies had a greater presence in contemporary IF and believe that the many digital examples that you provide hold promise. The one thing I would suggest is that an author consider alt-descriptions and other screen reader-friendly features for persons who rely on these technologies.
Since many platforms offer free hosing (WordPress, for instance), it would be easy to mock-up a page to be used in concert with a game. I think a mobile-friendly companion metatext could add a lot to a game, for instance.
Another idea: feature a playlist of songs to reflect a character’s personality or mood.
Neat topic!