Okay... I agree it is generally a bad idea and shouldn't be done but I just had to test this. I just connected neutral to ground downstream of a GFCI in my house and it tripped. I actually thought it would take some load to make it trip - it didn't or at least I didn't have to cause new load. If GFCI works by tripping when there is unmatched current between hot and neutral wouldn't adding ground as a path to take some of the current off of the neutral cause it to trip?
I'll admit the first time I tried it didn't trip and then I put a GFCI tester on that particular circuit and it appears I have a bad GFI, the tester won't trip it either - so I tested two others and they both tripped in the N/G connect scenario.