Hi,
Technically speaking, it is possible, but would require some changes from firmware, to handle the larger battery (there are some time limitations implemented which cut off charging if it lasts too long - in this case it would most probably happen).
Also, the battery should have an identical characteristic NTC temperature sensor on the battery for this to work.
I would suggest looking in to FMU126 (although it has a different modem which supports 2G and some 3G bands) - it has the same form factor as the FM3612 and contains a 1200mah battery.