One way to have a 3rd state (without XUL) is by using the DISABLED attribute of the check box. I haven't done it, but I think by turning it on through an event it will grey it out.
When the form is submitted, you will not receive any state for that checkbox, neither on or off since it is not set. But this peice of info can be used to tell it has been disabled, i.e. greyed out.
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