Will the honeycomb grill on an 09-11 4G TL fit a 2012 without major modification? I basically want it to be "plug and play".

 

No major modification needed and it is plug and play but you will need more than just the grill.
So long as you are okay with the cost.... you will need parts from an 09 - 11, grill, bumper, fog lights,re paint etc.
Several members have converted their 09/11 to the 12/14 front end. Maybe look around and see if any of them have their parts or see if someone wants to swap parts with you.