After some new 3d design work, and some calculations, it looks like i can get the "shunt" resistance up to around 4 to 5 uOhm's, which is a much better figure i think, and not using copper is going to help, and after doing the thermal calc's it looks like we don't need too (because the average current is much lower at only around 250A). I'm also happy to use a PGA in the system somewhere and have a "high range" mode where i accept a marked reduction in resolution in order to be able to not go overrange. The total energy used during these short high current periods is small, so offset and absolute errors here will not affect the energy sumation nearly as much as the much longer periods spent at lower currents
I'm also looking at non contact bus bar sensing using a Allegro hall effect sensor as a fall back, but i'd like to avoid this due to the additional mechanical design and build complexity it brings. I'll probably put on a seperate FFC header to run out to this sensor board, giving me options as to where it sits and if to include it or not.