Is there any way to get the label of a GtkCheckButton or GtkRadioButton to wrap when its parent container is resized small enough so the label won't fit on one line?
You can access button's child widget — it's a GtkLabel in default setup, — and set various properties on it, e.g. wrap. However, since there is no height-for-width layout negotiation in GTK+ 2.x, what you want exactly can be difficult / close to impossible to achieve. Note that you can also replace the button's child if you want.