From the example you link to, I believe you can afford for the user to download all the variations of the form, you only want to hide some parts unless some option is chosen. Is this right? The alternative is when the potential additions to the form are so varied or impossible to compute on client side that you need to use a query to the server to get the new elements that have to be inserted.

If so, I have a related example at Re: replicating blocks of code in perl and cgi. This one replicates form elements instead of hiding them, but the basic idea should be the same.