I had similar issues with this (we use VB.NET in our shop, it may be different in C#).
I found that in order to make the button handler fire, you need to associate the UniqueID of the button to the target argument of the postback, instead of the ID or the ClientID.
Not sure if this will help you.
I used the following script:
function postbackFromJS(sender, e) {
var postBack = new Sys.WebForms.PostBackAction();
postBack.set_target(sender);
postBack.set_eventArgument(e);
postBack.performAction();
}
Then in the codebehind (on PageLoad)
postbackButton.OnClientClick = String.Format("postbackFromJS('{0}', '{1}')", postbackButton.UniqueID, "")