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.