Implementing powerline communication on a battery-fed 12V line
You are right, the real question is what does the impedance of your 12 V power line look like at high frequencies.

I don't know, but this is something you can measure yourself.  Try to feed a signal onto the 12 V line thru a resistor (with cap to block DC but large enough to not add significant impedance) and measure the attenuation.

133 kHz sounds very low to me.  I would try around 1-2 MHz for starters.  With a good filter, you can get by with very little signal at the receivers.  After all, think of how little the signal is in the antenna of a AM radio.

It would probably help a lot if you can add even a little inductance to each connection of the 12 V line.  At 150 A, that would be big and expensive to actually buy inductors, but maybe just wrapping the feed cable around a few loops would help.  Three turns around a broomstick might make a difference.  The reason I said broomstick is because then it will essentially be a air core inductor, so you don't have to worry about saturation current.  1 &micro;H at 2 MHz has a impedance magnitude of 13 &Omega;.  1 &micro;H is going to be hard to achieve with a few loops of cable, but it is easy to mentally work from there.  100 nH, which might be possible, will be 1.3 &Omega;