Apparently there isn't one by default. At least, not when I'm trying to call it.
Just dumping the keys from UIManager.getLookAndFeelDefaults().keys()
produces the following if the key contains CheckBox:
CheckBox.foreground
CheckBox.border
CheckBox.totalInsets
CheckBox.background
CheckBox.disabledText
CheckBox.margin
CheckBox.rollover
CheckBox.font
CheckBox.gradient
CheckBox.focus
CheckBox.icon
CheckBox.focusInputMap
After reading akf's answer, I started digging through the UIManager code in the plaf.synth packages and found calls that essentially delegate the null disableCheckedIcon to the look and feel classes to try to convert the standard .icon to a grayed version. So I ended up with this:
Icon checkedIcon = UIManager.getIcon("CheckBox.icon");
Icon dsiabledCheckedIcon =
UIManager.getLookAndFeel().
getDisabledSelectedIcon(new JCheckBox(), checkedIcon);