i'll be honest, it has been kind of confusing when i've linked toots to non-masto using friends and the cw text was the only thing displayed in the description
my preferred solution would be for the external description to be "content warnings: (cw text)" and the external image to be the user's avatar
@Dex I believe the concern there is that that adds hardcoded text in one language (English in this case, but it doesn't necessarily have to be that) to previews, which could confuse those not familiar with that language.