Ok, so I got my semi broken A490 camera (which I managed to sort of fix, but some buttons won't work). I tried different input voltages (via the battery connector, not DC IN), and it worked quite OK at 4.3V
I didn't let it run for an extended period, but it seemed OK.
Of course, there is no guarantee that other 2AA cameras will work at this voltage, so try at your own risk.