How To Replace A Function Name Using JavaScript And Have It Be Recognized As A Function

I am trying to replace the JavaScript onclick event handler in ASP.NET that is added to a button control when using validation controls. This is what is output into the HTML from ASP.NET in this scenario:

<input type="image" name="ibSubmit1" id="ibSubmit1" src="button-green-submit.gif" onclick="showProgress1();WebForm_DoPostBackWithOptions(new WebForm_PostBackOptions("ibSubmit1", "", true, "Group1", "", false, false))" style="border-width:0px;" />

I have looked pretty estensively, and unfortunately there doesn't seem to be a way to modify the function server side before it is injected into the page.

Since I am developing a control and desire it to be non-invasive and self contained, and I am interested in obtaining the validationGroup parameter of the WebForm_PostBackOptions object, it seems that the easiest solution would be to use JavaScript to replace the WebForm_DoPostBackWithOptions function name with my custom wrapper function and leave all of the rest of the parameter information intact - then I can extract the information I am interested in, call my custom functions, and then forward the call on to WebForm_DoPostBackWithOptions.

NOTE: I am using jQuery to build my custom function, so if there is an easier way to do this with jQuery it is an option I will consider.

Here is the code I tried to replace the onclick event handler (not working):

$('[onclick*=WebForm_DoPostBackWithOptions]').each(function() {
var txt = this.onclick;
txt = txt + '';
txt = txt.replace('WebForm_DoPostBackWithOptions','ml_DoPostBackWithOptions');
this.onclick = eval(txt);
});

Using alert(), I verified that the text is being changed correctly, however whether or not I use the eval() function, the onclick handler doesn't seem to recognize it as JavaScript.

I thought of using a regular expression to get the validationGroup value, but this seems like it will be far more elegant and flexible if I can get it working...

Note: If there is a way for my control to interrogate the page it is on to find all of the buttons that will post back (regardless of what type of buttons they are) so I can retrieve the property server-side, this is also something I will consider.