I've got via an anonymous developer a special modified build of the reference jm 7.3 commandline h264 encoder, which takes an .avs yv12 (DVD-colorspace) input and of course outputs .avi.
I believe Roberto will be hosting the binary of this in Brazil soon.

Usage of this encoder is simple:

In encoder.cfg file you can see that by default it retains constant quantizer 24. By changing this file you can adjust encoder options.

Further discussion of this is continued on the HA's video section, here.