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.