The IsCharacterKeySet parameter is not [directly] exposed through the Object Model, so you can’t get at the parameter through the Parameters or even NestedObjects. Instead, you can access the IsCharacterKeySet parameter with the GetValue and SetValue commands.

XSICollection does, however, provide an Object Model way to get at the parameter:

LogMessage( isCharKeySet( Selection(0) ) );

function isCharKeySet( o )
{
	var oColl = new ActiveXObject( "XSI.Collection" );
	oColl.items = o.FullName + ".IsCharacterKeySet";
	return ( oColl(0) != null && oColl(0).Value == true );
}