Craig,
you can isntall an addditional Platform instance on another Windows Server with IIS as it does not require any additional Docuware server licenses and direct your RO users to use PlatformRO website on that server.
The only thing is - if you want users to be using Desktop Apps that require user licenses such as C2O and SmartConnect on RO site, you'd need to adapt web.config file on that server to set ReadOnlyMode=True