Hi Jake, a lot of rendering programs will have an option to "freeze" or capture the output from displacement mapping as polygon geometry which could then be exported to an STL file, so that's why using a rendering program could still be a possible technique. Not for generating a rendering actually but just for using their displacement and polygon manipulation tools.

You might want to check out Sculptris which is a particularly easy to use organic sculpting program which uses brush-based displacement modeling similar to ZBrush:
http://www.pixologic.com/sculptris/emails/operating-system.php

- Michael