The GtkCellRendererPixbuf class implements the GtkImageCellAccessible type, which implements the AtkImage protocol, and it can provide a “description” property, so it’s already possible to tie a textual description to a pixbuf renderer in a tree view.
I’m a bit fuzzy on how to go from a property in the accessible cell renderer object to, say, a column in the model.
Edited to add: There is currently no way to bind a property on the accessible object tied to a cell renderer to a column on the model, and it would still be hard to achieve—especially in an API/feature frozen GTK 3.
First of all, you should ask the accessibility developers for their opinion. Adding this functionality to GTK 3 is going to be complicated, I’m afraid; there are known scalability and performance issues when it comes to accessibility of tree views.
We cannot add an accessible object for every cell with a description in the tree view, because that could end up creating hundreds of thousands of object at the same time, which would swamp any accessibility consumer. Maybe we could create a temporary object for the row currently being selected, but that would require bridging knowledge of the accessibility layer into the data model used by the tree view, which may simply not be possible.
This requires a lot more discussion before opening a new issue.