Ben,

Have you considered using a Dedicated Waterside Economizer? It allows you to simulate a chilled water loop serving a cooling coil, and a cooling tower linked to the chilled water loop via a heat exchanger. The setup is limited to one cooling tower per cooling coil, so it might not work with your project; setup; but it does allow for backup cooling from a chiller. See section 2.13 of the ApacheHVAC User's Guide for more info.