Disable ribbon button through onload JavaScript

CRM 2011 is giving full control of custom button through ribbon concept over ISV configuration concept of CRM 4.0. Here I have mention how to enable/disable button according to rules we define.

Here, rule is a straightforward one. For example, if we need to disable a button depending on logged in user’s team, I am still unable to address through a rule definition. Till I find it I found a way of doing it through onload JavaScript.

Ribbon item behaves according to a defined style. What I do is manipulating the style according to my need. Here I need to disable a particular button if the user is not in “Admin” team.
First I grab the id of the button. I assume you know how to do that by F12 function as shown below.

Now replace the standard style of that button by standard style of the disabled button as required.

if (!userIsInTeam("Admin"))
{
 if (window.top.document.getElementById('<ID>'))
 {
 var _id = window.top.document.getElementById('<ID>');
 var str = _id.outerHTML;
 var str1 = str.replace("ms-cui-ctl-large", "ms-cui-ctl-large ms-cui-disabled");
 _id.outerHTML = unescape(str1);
 }
}

I urge this is not the best way, but this works.
If I manage to find a way to define a rule in XML, I am publishing it. If you find first, please share it for me too.