I am not seeing how a second G-N bond in a genny would affect GFCI operation? I certainly do not know all of the reasoning behind only having one bonding point, but one reason certainly is that grounds are typically smaller than neutrals-and with mutliple bonds a lost neutral results in the ground carrying neutral current-which would lead to overheating, etc.
In a "typical" 5kw to 10kw backup situation, we usually use SO cord between genny and premisis-so ground and neutral are the same size. Ohms law says in this situation, both wires will carry the same current-a violation of the NEC rule against "objectional current" on a ground.
However, if you draw the circuit out, the ground and neutral are in fact parallel conductors, so you really have a 3 wire system from the genny to the house-not unlike the 3 wire system from the POCO transformer to your house. POCOs often feed multiple homes with 3 wire from a transformer and GFCIs still function on the home. Since the genny GFCIs will get neutral and ground from the common G-N bond in the genny, each receptacle would act like a receptacle in a different home on the transformer.
There would be somewhat of a safety hazard if both the neutral and ground conductors had a bad connection, in that the genny frame could become energized to a level above ground, since they are typically sitting on rubber/pvc wheels and pads.
As for grounding electrodes, keep in mind that if a ground rod does not measure less than 25 ohms to ground, a second rod or electrode is required. Curiously, if two electrodes exist, code doesn't care what the resistance to ground is. Also, Mike, code does allow "plate" electrodes min of 2 square feet exposed to soil, must be 1/4" thick, and buried below permanent moisture level "if practicable" . I would argue that for a temp service -ie weekend, burying is impracticable and surface mounting with artificial (if needed) moisture should be equivalent? Plate electrodes also require 2 unless resistance to ground measures less than 25 ohms. (NEC 2011 250.52 (A) (7) and 252.53 (A)(1) and (2))