This is not exactly what you're asking for (media inside media), but it's really close in spirit (nested narratives), and I really like it: a book written in Portuguese in the XIX century, called Noite na Taverna (Night in the Tavern).
The book has an overarching story of friends telling each other stories in a tavern, over booze; with all those nested stories being about love, despair, and death (it has a strong gothic vibe).
And, as each character tells the others a story, there's always that fishy smell that the story might be actually bullshit; and other characters do raise some doubts about its in-universe veracity (like Bertram does to Solfieri). And you, as the reader, do the same - but in no moment you question the veracity of the overarching story, and you feel like you're inside the tavern alongside the drunkards.
So it's a lot like the author is toying with your suspension of disbelief - redirecting it from the overarching story to the nested stories, and as you doubt the later you get even more immersed into the former.
If I must use an example of media within media, then my choice would be "The Book" within Orwell's 1984. I think that it's a great piece because it shows Orwell's views on politics and society, while still serving narrative and worldbuilding purpose - for Winston it's a material proof of the Inner Party's bullshit, for O'Brien it's a tool of the Inner Party to sniff out dissidence. (Note: 1984 is extremely misrepresented nowadays, I'm aware, but I still like it.)
I've never noticed this usage of the past tense in the appendix about Newspeak - you're right, it does. And it's also written in standard English, so interpreting it as written in a world after Oceania fell is viable.
And following this line of thought we could even interpret the main story as a narrative within another.
Another possibility is that the appendix is not written in-universe, and uses the past tense because it's how people expect storytelling to be written in English, with Orwell speaking directly to the reader instead of Winston Smith.
It's definitely not Winston, he died in the main story. I interpreted it as an unnamed historian writing about this years later. I don't think Orwell would self-insert and write from an in universe perspective.