Emulate Postback of "All Day Event" in custom list form

I don't think you can.

The other option to modify the form is to hide fields with javascript. Revert your new/edit form to the default web part. In the snippet you will find a function that searches for form fields and changes their "display" style to "none". With SPD add at the end of the file the code and call the document.getElement... line for every field you would like to hide.
The title of the element is usualy its display name. To be sure - find the title of the element by viewing the source of the add/edit page and search for the name of the column.

document.getElementById(get_form_elem_id("Title of element 1").style.display='none';
document.getElementById(get_form_elem_id("Title of element 2").style.display='none';
 
function get_form_elem_id(title)
   {
 
	//take the form elements collection
    var elem = document.forms[0].elements;
 
	//do a loop
	for(var i = 0; i < elem.length; i++)
	{
 
	  //if the title is equal to the searched title
	  if(elem[i].title==title)
	  {
	    //return the id of the searched element
	    return elem[i].id;
	  }	  
	} 
	return false;
   }

Select allOpen in new window

That only partially worked.  Your solution took care of the field only and only if it has a Title.  I actually found a better solution: http://moblog.bradleyit.com/2008/10/disabling-workspaces-from-sharepoint.html?showComment=1224610080000

It works great.  Thanks for you help anyway.