I'm fairly new and haven't worked with a pro post guy yet. I know about wanting to get the best signal to noise ratio, but as far as out side:
When The location of the actors facing and the position of the boom makes a sound source like a freeway sound different, for different shoots, is there anything else besides maybe a Sound Blanket, that I should have done to help out post mixing?
Would they, in this case, gate the dialogue and eq so the dialogue sounds similar, and then use room tone as a constant background?
Any where to find a post mixer to talk to or read about these particulars?
As a sound person, I would lobby against any location that made good dialog matches impossible. I normally tell the director what problems I hear, let him/her hear what I'm hearing in my phones so they can hear the problem, and explain the problem in simple details. If the location can not be altered, you get what you get and try to fix it in post, maybe even with ADR.
The use of sound blankets or other grip gear to minimize the background noise is certainly a possibility. Instead of blocking the sun or other lights, you're blocking sound. It's more difficult with sound, but it can help.
Gating may or may no be a good option in post, depending on the amount of background noise.
Yes, adding some noise to mitigate the differences can help, but only so much.
Without knowing the logistical particulars, it's difficult to say if that would work.