I recall that a company was marketing a "binary trigger system" for the AR platform that incorporated a modified fire control group with a custom grip (patterned off of the Magpul MOE family of grips) that housed all of the electrical components. If I remember correctly they passed the controls through the existing hole for the handgrip retention screw.
As for the chip itself, I believe that the xEM would be ideal. On idea for antenna placement would be a "swelled" backstrap. But it would necessitate implatation further forward towards the edge of the webbing. Possibly creating a pinch hazard.
Another reason for the xEM would be the ready modifiable projects and products that already exist.
Not to mention the xEM being inheritnly less expensive.
However, I would be interested in possibly developing a system comparable with VivoKey.
My main reasoning for this is the the antenna size.
Something like a Flex series transponder will obviously have less issues with read range and orientation. Not to mention the range in implant location and it not being a "single use" implant.