How to disable button generated by submit_to_remote?

I got a submit_to_remote button that I'd like to show as disabled on a
certain condition. I'd assume the usual :disabled => true would do but I
have not been able to. Comparing page source shows that for example:

<%= submit_to_remote "add", "Add", :url => { :action => "add_groups" },
:update => 'divgrouptable', :disabled => true %>

the ":disabled = true" statement has no effect on the generated html.
Also tried "{ :disabled => true }" and a few others.

I went as far as writing on-load hook and disabling the buttons in the
java script as shown below but sounds like a lot of effort to achieve
this. How could I make this less "obtrusive"?

function disable_buttons()
{
<% if @readonly %>
   document.getElementsByName('add')[0].disabled = 1
   document.getElementsByName('remove')[0].disabled = 1
<% end %>
}

</script>

<body onload="javascript:onloadd()">

<original form goes here.......>

</body>

These 2 versions will disable the button:

<%= submit_to_remote "add", "Add", :url => { :action =>
"add_groups" }, :html => {:update => 'divgrouptable', :disabled =>
true} %>
<%= submit_to_remote "add", "Add", :url => { :action =>
"add_groups" }, :update => 'divgrouptable', :html => {:disabled =>
true} %>

I have never used the submit_to_remote before and I'm not too familiar
with the Ajax helpers so I'm not sure if :update has to be inside
the :html hash or not. In any case, in the docs the example reads like
this:

:update => { :success => "succeed", :failure => "fail" }

I don't know if your :update stuff will work as it is.

Both examples work, thanks.