Groovy Grails - how to read body contained within tag?

Question

I'm considering reading the body of a tag to determine what variables the tag should send to the body.

e.g, here's the GSP

<g:get_domain_info id="${patientInstance?.id}" >
  <tr class="prop">
    <td valign="top" class="name"> ${firstNameLabel}</td>
    <td valign="top" class="value"> ${firstNameValue}</td>
  </tr>
</g:get_domain_info>

The tag somehow reads the body, sees that it wants "firstName", so outputs the first name label and value to the body.

Is this possible?