LEFT or UP arrows would select the previous enabled button in the group.
RIGHT or DOWN arrows would select the next enabled button in the group.
Optionally, HOME & END would select the first or last enabled button in respectively.
This functionality could be added to the existing org.apache.pivot.wtk.ButtonGroup class, or a new class named RadioButtonGroup could be created.
The ButtonGroup would track the order that the buttons were added and use it to select the appropriate button.