Thanks.
I was a little confused, because the lede sentence was "Ropey is a utf8 text rope for Rust, designed to be the backing text-buffer for applications such as text editors."
Pretty much all text editors are expected to implement decorations and references, somehow, and some popular text buffer APIs support those.
If you'd like an example of how this can be done, Swift's AttributedString type is exactly that. It manages the association of runs of attributes (font, kerning, color, etc.) with Unicode text, and its backing storage uses a rope type provided by the swift-collections package. (The rope module itself isn't stabilized yet, so it's really only used for AttributedString at this point, AFAIK.)
https://github.com/swiftlang/swift-foundation/tree/main/Sour...
https://github.com/apple/swift-collections/tree/main/Sources...
For data that isn't part of the document, you could use a write-through wrapper around the rope, with a secondary data structure mapping ranges of the document to this extra data.
From the wrapper's point of view, there's no difference between character and non-character data, and the whole buffer can be modeled as a collection of indices mapping ranges of the document to different kinds of data.
One of those indices could be a rope (mapping document ranges to character data, for the document text). Other kinds of indices could also be used. The important thing is that all edits go through the wrapper so that all the relevant indices get updated.