In order to access a custom/user-defined node, you just have to concatenate the XML-xpath into one identifier and use it within the getPropertyViewHelper. For example:

<immobilie>
  <kontaktperson>
    <email_direkt>mail(at)example(dot)org</email_direkt>
  </kontaktperson>
</immobilie>

{oi:getProperty(object: immobilie, index: 'kontaktperson_emailDirekt')}

or:

<immobilie>
  <verwaltung_techn>
    <user_defined_simplefield feldname="reserviert">ja</user_defined_simplefield>
  </verwaltung_techn>
</immobilie>

{oi:getProperty(object: immobilie, index: 'verwaltungTechn_userDefinedSimplefield_reserviert')}

In the identifier each XML nodename is divided by a _. If the nodename contains a _ it will be transformed to lower-camel-case.