Is there a way to change (for the purpose of localization) the null placeholder in a binding in Cocoa?

The bindings are set up in Interface Builder for a popup button. The two-way nature of the bindings as set up in IB is needed, so doing it programmatically is not really appealing.

I am aware that the standard way of handling localizations of a nib file is by making one for each language, but since this is the only difference in the whole nib file between the language versions, it seems a bit excessive for a single string.

If there is a way to modify a binding created in IB, I was thinning about doing it in the file's owner's awakeFromNib method.