This won't work, as onmouseover does not fire on disabled inputs, in most browsers. (Opera is an exception, and IE usually doesn't, except when you trick it with a drag operation.)
In any case it sounds like a pretty weird and user-unfriendly trick to me. Why do you want it disabled? If it becomes non-disabled when you try to interact with it there doesn't seem to be any point to it (but plenty of accessibility downside).
If it's just a styling thing, then use styles:
<style type="text/css">
.weirdinput { color: silver; }
.weirdinput:hover, .weirdinput:focus { color: black; }
</style>
<input class="weirdinput" value="smth" />
However, IE<8 doesn't support :focus, and IE<7 doesn't support :hover on non-links, so if you need it to work on that you would have to add some scripting, eg. something like:
<style type="text/css">
.weirdinput { color: silver; }
.weirdinput:hover, .weirdinput:focus, .weirdhover, .weirdfocus { color: black; }
</style>
<input class="weirdinput" value="smth" />
<!--[if lt IE 8]><script type="text/javascript">(function() {
// Fix focus/hover for IE on all inputs with class 'weirdinput'
//
var inputs= document.getElementsByTagName('input');
for (var i= 0; i<inputs.length; i++) {
var input= inputs[i];
if (input.className.indexOf('weirdinput')!==-1) {
input.onmouseover= mouseover;
input.onmouseout= mouseout;
input.onfocus= focus;
input.onblur= blur;
}
}
function mouseover() {
this.className+= ' weirdhover';
}
function mouseout() {
this.className= this.className.replace(' weirdhover', '');
}
function focus() {
this.className+= ' weirdfocus';
}
function blur() {
this.className= this.className.replace(' weirdfocus', '');
}
})();</script><![endif]-->
