Hi,
Yes, MSDN documentation also confirms that javascript option is not available for <DisplayRule>.
http://msdn.microsoft.com/en-us/library/gg334209.aspx
While <CustomRule> is available for <EnableRule> to work with javascript function.
http://msdn.microsoft.com/en-us/library/gg328073.aspx
But Here is a workaround to achieve it.
http://nishantrana.wordpress.com/2012/05/18/hiding-ribbon-button-using-javascript-in-crm-2011/
Please mark this post as an answer if this helps.
Ajay Chawla