Customisable forms per node

I'm no expert, and I'm sure there's a better way to do this, but I have a working solution in 7.x dev.
Create a taxonomy for your "options" (additional form elements such as number of attendees, additional info, food allergies, etc)
In your event content type, add this taxonomy field with checkboxes to select if you want to add the additional form elements or not.
In signup_form.inc, create a script to check which taxonomy ID(s) are present, then add the corresponding form elements.

This script collects TIDs for field "field_event_signup_fields" and stores them in array $result:

   $num = count($node->field_event_signup_fields['und']);
    $result = array();

  if ($num > 0) {
    for ($row = 0; $row < $num; $row++) {
    $result[] = $node->field_event_signup_fields['und'][$row]['tid'];
    }
}

Then create form elements for each option:

if (in_array('124', $result)) {
    $form['signup_form_data']['num_total'] = array(
    '#type' => 'textfield',
    '#title' => t('Number of Attendees'),
    '#size' => 3, 
    '#maxlength' => 2);
    }

Again, I apologize for the elementary PHP logic, but I hope it helps.