PyGTK Entry widget in TreeViewColumn header

How can I make a gtk.Entry widget focusable or editable within a gtk.TreeViewColumn header/title? I've tried this:

# Create tree-view.
treeview = gtk.TreeView()

#...

# Create column.
renderer = gtk.CellRendererText()
column = gtk.TreeViewColumn(None, renderer, text=0)

# Set column header.
header = gtk.VBox()

title = gtk.Label("Column")
header.pack_start(title)

filter = gtk.Entry()
#...
header.pack_start(filter)

header.show_all()
column.set_widget(header)

# Add column
treeview.append_column(column)

But the Entry widget in the column header is not editable and will not focus. I've tried setting 'clickable' to both True and False. I'm using pygtk 2.21.0-0ubuntu1 and libgtk 2.22.0-0ubuntu1 on Ubuntu 10.04. Any help would be greatly appreciated.

EDIT:

The issue stems from how a GtkTreeViewColumn header is displayed. The header widget is placed inside a GtkAlignment whose parent is a GtkHBox whose parent is a GtkButton whose parent is finally the GtkTreeView. The GtkButton is intercepting and preventing my GtkEntry from being focused and receiving mouse input.