Is the tree item that hosts the control in a column (and not the first one)? Can you select the right item with a standard tree item locator? If not, a possible work around might be to select an item in the right tree row and navigate to the correct column using the keyboard (for instance by sending a TAB). Having done this, the combo should be active at which point we need to identify it... (can you name it?)
It would be great if you sent this as a feature request to:email@example.com
where we can properly schedule research and implementation.