Yes, the splitter panels can contain any HTML element.

If the element isn't a server running element then you can access it using javascript like:
   var myElement = document.getElementById('myElementID');

Otherwise, if the element is a server running element (runat = "server"), the client id that you set to it 
might change on client side, so the best way to access it using javascript is like:
   var myElement = document.getElementById('<%=myElementID.ClientID%>')