How to set grid column header caption?

I have a checkbox field with a long name, so when I add this field to a grid, the column takes up a ton of unnecessary space.  It seems the client-side API supports access to the grid's columns, rows and cells collections, but not to the grid's header objects.  I've tried the following JavaScript (added to Custom Content) on the Opportunity Grid.  When I click Find on the grid, the alert correctly pops up and displays Assigned To, so I know the 1st line of the function is correctly capturing the header text.  The problem is that the 2nd line is never called as the header caption remains unchanged.  Any thoughts or suggestions?  Thanks.

     <script type="text/javascript">
               var o = document.getElementById("_Captoppo_assigneduserid").innerHTML;
               alert("o=" + o);
               document.getElementById("_Captoppo_assigneduserid").innerHTML = "PM";