In Softimage 2013, you can use the ViewportCapture.DSCodec parameter to set the codec for your viewport captures.

http://vimeo.com/39630053

DSCodec is an string that encodes the codec ID and parameters. To get the DSCodec value, do a viewport capture and set the Codec. Softimage will log the DSCodec value in the script history:

# INFO : ViewportCapture.DSCodec: AAAAFnNwdGxycHphAAAAAAAQAAACAAAAABR0cHJsAAACAAAeAAAAAAAYAAAAGGRyYXQAAAAAAAAAUwAAAQAAAAEAAAAACW1wc28AAAAADG1mcmEAAAAAAAAADHBzZnIAAAAAAAAACWJmcmEAAAAACm1wZXMAAAAAABxoYXJkAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAKc2RuZQAAAAAADGNtZnJhcHBsAAAAAA==

Here’s a Python snippet that shows how to set the DSCodec parameter:

from siutils import si
si = si()					# win32com.client.Dispatch('XSI.Application')
from siutils import log		# LogMessage
from siutils import disp	# win32com.client.Dispatch
from siutils import C		# win32com.client.constants

def dispFix( badDispatch ):
	    import win32com.client.dynamic
	    # Re-Wraps a bad dispatch into a working one:
	    return win32com.client.dynamic.Dispatch(badDispatch)

oViewportCapture = dispFix(si.Dictionary.GetObject( "ViewportCapture" ))
oViewportCapture.NestedObjects("File Name").Value = "C:\\test.mov";

oDSCodec = oViewportCapture.NestedObjects("DSCodec")

#log( oDSCodec.Value )
# INFO : ViewportCapture.DSCodec: AAAAFnNwdGxycHphAAAAAAAQAAACAAAAABR0cHJsAAACAAAeAAAAAAAYAAAAGGRyYXQAAAAAAAAAUwAAAQAAAAEAAAAACW1wc28AAAAADG1mcmEAAAAAAAAADHBzZnIAAAAAAAAACWJmcmEAAAAACm1wZXMAAAAAABxoYXJkAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAKc2RuZQAAAAAADGNtZnJhcHBsAAAAAA==

oDSCodec.Value = "AAAAFnNwdGxycHphAAAAAAAQAAACAAAAABR0cHJsAAACAAAeAAAAAAAYAAAAGGRyYXQAAAAAAAAAUwAAAQAAAAEAAAAACW1wc28AAAAADG1mcmEAAAAAAAAADHBzZnIAAAAAAAAACWJmcmEAAAAACm1wZXMAAAAAABxoYXJkAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAKc2RuZQAAAAAADGNtZnJhcHBsAAAAAA=="