Hi I assume you mean the _User_ Interface; the "Renderman Interface" is generally a term that is used to refer to the specification. PRMan itself has no UI; the *TOR product is a wrapper around PRMan, so yes, PRMan is definitely useable without the UI, e.g. from a shell script.
PRMan is not DOS-based. Under Win32, it primarily runs as a set of console applications, just as it does on Un*x.