Libav provides a native encoder for prores.
avconv -i source -c:v prores -an out.mov
The proxy, lt, standard, hq and 4444 profiles are available.
avconv -i source -c:v prores -profile:v hq -an out.mov
The encoder by default uses an optimized matrix to provide the best quality, is it possible to use one of the standard matrices (the name match the profile names) instead.
The private options -bits_per_mb and -qscale have impact over the overall file size and resulting quality as well the expected -b:v option.
Passing -qscale and not setting the bitrate constraint results in a faster encoding.
avconv -i source -c:v prores -qscale 4 out.mov