First thing is to make hidden beacons on top of all of the assemblers, effect extending 1 out in each direction.
Problem is that each assembler will be affected by its own beacon. So to fix that you could make a 4-color checkerboard (pic) and have 4 assembler protos and 4 module types. An assembler with its center on a red square gets a red module, which only affects non-red assemblers. Same for other colors. Neighboring assemblers will always have a different color at the center so they'll affect each other. (You need at least 4 colors for square assemblers larger than 1x1.)
Second issue is that you want the neighbor bonus to only apply when a machine is active. To do that you could make a proxy container for the beacon module inventory on a hidden surface, and connect it to inserters with circuit conditions. When assembler is inactive, an inserter moves the module out of the beacon module inventory. When assembler is active, another assembler moves the module back into the beacon inventory.
safe cliff