Update 12/12/2015 - Just stopping by again to confirm myself that this method is still working even from recent patches, also confirmed by various people
Edit: you must first level up your chomper to level 3 to unlock the snare trap.
This can be done in Garden Ops mode, just load up any map on easy and place a snare trap in the path of a zombie. If they die in the snare it counts as a "swallow" kill as long as it was a snare damage that killed them. hope this helps :)