NOTE: I am aware of two independent projects aimed at creating a free, RenderMan-compliant renderer: GNU Man and Freeman. Only the first one released some code, which is still in a very early stage. Hopefully we will enjoy a working implementation in some future. There is an implementation called BMRT by Larry Gritz, which is not free software, but is available free of charge, also for commercial use. If you know of any free RM-compliant renderer which is free software, please let me know.
UPDATE: Finally there is a very promising renderer: Aqsis. Please check it out, I haven't tested screenhack with it yet.

Creating MPEG movies is very simple. First, you have to render frames from your screenhacked RIB:

$ rendrib mymovie.rib

You should have a directory with several tiffs, like:

f0001.tiff
f0002.tiff
f0003.tiff
f0004.tiff
f0005.tiff

Then you create a .param file, like this:

PATTERN IBBPBBPBBPBBPBBP
# this is where our mpeg will go:
OUTPUT mymovie.mpg
BASE_FILE_FORMAT PNM
INPUT_CONVERT tifftopnm *
GOP_SIZE 16
SLICES_PER_FRAME 1
# this is where our tiffs are:
INPUT_DIR /home/me/movies/
INPUT
# and here we give how many frames we have
f0*.tiff [000-250]
END_INPUT
PIXEL FULL
RANGE 10
PSEARCH_ALG LOGARITHMIC
BSEARCH_ALG CROSS2
IQSCALE 4
PQSCALE 5
BQSCALE 15
REFERENCE_FRAME DECODED
BIT_RATE 1000000
BUFFER_SIZE 327680
FRAME_RATE 30

...and run the command:

$ mpeg_encode demo.param

You should have a nice MPEG movie. Now you can add sound, effects etc. with Broadcast 2000, MainActor or similar tools.