I noticed that in Firefox I can add a #MyAnchor tag to the action attribute like...
Just a guess, but have you tried using the page + the anchor name.
<form action="mypage.aspx#MyAnchor">
I've used this to retain the fragment across postbacks:
var f = document.forms[0];
var index = f.action.indexOf("#");
if(index>0)
f.action = f.action.substr(0,index) + "#" + tabId;
else
f.action += "#" + tabId;
You can deal with this either on the client side or on the server side:
On the server side: add a hidden element with the anchor as the value and do a redirect on an URL build on the server.
On the client side: jQuery for instance allows you to serialize a form's parameters into a URL; you would just need to append the anchor and assign this to window.location.