This script will render out via "Preview - Grab Viewport" for multiple cameras at once. Useful if you create preview cameras a lot, or need to make multiples at once.
Installation:
1. Run script
2. In Max goto "Customize->Customize User Interface...->Toolbars tab->Category: ColinScripts->OpenGL Previews" and drag "OpenGL Previews" to a toolbar, click to run.