The goal is to allow a function to be called from within an ASPX imagebutton control that is used as part of a form. The need for an absolute URL is that Facebook Canvas applications require them. So the rendered control should look something like
<input type="image" src="http://myURL.com/images/submit.gif" />
The ASPX markup might be something like <asp:imagebutton runat="server" imageurl="<%= MyAbsoluteURLFunction("images/submit.gif)" %>" />
My sample syntax is obviously incorrect, which is why I'm here. The reason I want to use an ImageButton is because it will have events attached to it.