<p>I want to write an <code>onClick</code> event which submits a form several times, iterating through selected items in a multi-select field, submitting once for each.  </p>

<p><strong>How do I code the loop?</strong></p>

<p>I'm working in Ruby on Rails and using <code>remote_function()</code> to generate the JavaScript for the ajax call.</p>

My quick answer (as I've not coded it yet) would be to create another function that creates a POST using XMLHTTPRequest and the specific parameters for a single call. Then inside your onClick() handler call that function as you loop through your selected items.
I would suggest that you do a Proof of Concept just using a dummy HTML page and javascript and then try to figure out how to get it to work in RoR.
Also, why are you attempting to make the multiple calls from the browser as opposed to handling the looping conditions in the RoR controller?