Interim update:

The brown stuff seems to give more, not less contrast. However it is also more susceptible to "fog", and must be coated, dried and exposed faster.

I'm going to try diluting the mix a little more, as it seems very "thick". I might also add a little more silver to the diluted mix, and a fraction of a drop of 3% hydrogen peroxide if fogging continues to be a problem.

Postcard list members may get to see the results of my experiments, as I'm testing on postcard stock