Accessible text in a GtkTreeView?

[Dylan McCall <dylanmccall gmail com>]

I'm working with a tree view widget which needs to be nice and
accessible. Here is a picture:
http://ubuntuone.com/1pP5oWD1HCIUQ4OqptHZfw
The Install column has three cell renderers, and many of the columns
have content that would be unclear coming from a screen reader.
Currently, without any special accessibility code for our tree view,
Orca reads rows like this:
"Image, Document Viewer, collapsed, 476 kB"
"Image, Image Viewer, collapsed, Tick mark"

What I would _like_ it to say is something more like this:
"Image Viewer, Install checkbox ticked, Collapsed, Downloaded"

Here are the problems I have:
In the column with three GtkCellRenderers, the only one Orca mentions
is the GtkCellRendererText with the package name. The cell with the
checkbox is completely hidden to it, even when it has the keyboard
focus. Can I control which of these appear?
How can I get Orca to _not_ mention the image column on the left
which, 90% of the time, does not show an image?
I don't know how to set accessible text for a column. We use a tick
mark to indicate that something is downloaded, but if it's being
spoken it makes a lot more sense to just say "Downloaded."

So, where do I start if I want to control accessibility for a tree
view? Is there a handy API for this?

Thank you,

Dylan