a simple way to render H264 content directly from Cinelerra

Use presets!
The FFmpeg encoding guide has excellent information on using presets:
http://ffmpeg.org/trac/ffmpeg/wiki/x264EncodingGuide

At the time of this writing, by running:

ffmpeg -i input -c:v libx264 -preset -tune dummy.mp4

I see that the available presets are:
ultrafast
superfast
veryfast
faster
fast
medium
slow
slower
veryslow
placebo

So really, all you need to do is translate that simple one liner into the Cinelerra YUV4MPEG Stream:
 ffmpeg -y -i - -threads 12 -c:v libx264 -preset medium %

Like so:

WahLah!  Easy-peasy conversion to H264.

CM

Reference:

FFmpeg x264 Encoding Guide