GtkFileChooser: Add save label size group?

[Christian Neumair <chris gnome-de org>]

Some apps need to attach extra widgetry to the save dialog which often
visually looks like:
"Label: " GtkComboBox1
"Label2: " GtkComboBox2
The problem is that according to the HIG all those labels should have
the same width, even those that are above the extra widgetry:
"Name:" GtkComboBox: filename
"Save/Create in folder:" GtkComboBox: save folder
Unfortunately, the labels aren't accessible to the public and therefore
app authors can't simply create a custom size group that contains both
default labels and their custom ones. Ergo it seems to be a good idea to
add a GtkSizeGroup that enables app authors to simply to do
gtk_size_group_add_widget (chooser->save_label_size_group,
customwidget);
to align with the HIG.
Any ideas on how to implement it?

regs,
 Chris