How do you set accept headers in remote_function?

How do I set the accept header for an AJAX request from the
remote_function helper? I've tried everything I can think of but no
dice.

<%= remote_function(
# ...
:requestHeaders => { :Accept => "application/xml" },
# ...
) %>

What I want:
new Ajax.Request(url, {
  ...
  requestHeaders: { "Accept" : "application/xml"},
  ...
});

Thanks in advance