Planning an accessible dynamic form with Prototype

Hi everyone,

I have a bunch of different data types with some things in common. So
I'm creating a model to hold all of the fields and creating subclasses/
using inheritance to delineate the relationship between all of the

So far so good. I'm finding building the new form a bit more
challenging...because I have a select list of the different subsets of
the model. If the user picks one of those options, the form is
supposed to add the fields specific to the that option to the general
fields. I'm not sure this is clear...I basically want a select box
that adds fields to a general form depending on what the user picks.

I'm not sure how to implement this. My first thought is to use
fieldsets or hidden divs and use toggle to show or hide a given part
of the form depending on what the user picks. A particular selection
would just toggle the appropriate, hidden part of the form.

This would be fine with me. The trouble is that if a user has a
screen reader, the reader will read all of the sections (to the best
of my knowledge). This leads me to wonder if there is a more
accessible way to do this. Is there another, AJAXy way I can update
the form's fields to reflect the User's choice?