StringResourceModel with a defaultValue as model

Hi all,
is there any way to implement a StringResourceModel which allows to give
a defaultValue as model?

First I tried to override getDefaultModelAsString() of Label. (is final)
Than I tried to implement an extending
ModeledDefaultValueStringResourceModel, catching a
MissingResourceException on getObject() or load() and retrieving my own
Model<String> defaultValue, but everthing is final here, too.

So, Im not able to achieve this, right now.

I would like to do this, because I have a dynamic way of giving buttons
a label depending of my Wizard-subclass (have my own
SpecialWizardButtonBar).

And if a button label is not availabe I want to fallback somewhere, but
the default cannot be a StringResourceModel.

Can someone help? Or is this a feature request and have to open a ticket?

kind regards, nice weekend
Patrick