I would be willing to bet all the RPi boards use a single shared ground net. HOWEVER, between any two points on the circuit board that are nominally "ground" there is a finite, non-zero resistance. If you pass significant current through any part of the R-Pi board, you are responsible for the local power dissipation, and if reaches a significant number of amps, you may have local hotspots at constriction points, vias etc. that exceed the design rules for the board in that spot.
I would not recommend passing anything more than 3.1 A (total, including board and USB device power) through any part of the board, because that is the design power. At some (unknown) level of high current, you will have a voltage offset that disrupts normal operation, and/or a constriction area (likely a via) will overheat and damage some components or the board itself.