RJS and EXTjs component update (RAILS 1.2.3)

I want to use an RJS view to update EXT Panel bodies. But since the
panel bodies
have generated element ids, I can't see how to use
'page.replace_html'.

So instead I wrote a javascript function 'updateBody'.

In the RJS view, I want to say something like:
  page.call 'updateBody', 'some_panel', :partial => 'my_partial'

I looked at the source for replace_html, and then tried:
  page.call 'updateBody', 'some_panel', page.render(:partial =>
'my_partial')

but that doesn't work, and neither does:
  page.call ..., render_to_string(...)

Rendering the partial in the controller
  @rendered_partial = render_to_string :partial => 'my_partial'
does work, but it seems that there should be something easier.