bookworm
I think the point about FLARM is that it is very popular in parts of Europe so it has an installed base. However the CAA have rejected it and given a full explanation as to why. I have never seen this refuted, and as far as I know FLARM is off the table.
wigglyamp
I do not think CS23.561 would apply to most of the aircraft with no electrics. Most would come under CS-VLA or Section S which I think uses lighter loads. In LAA land you could certainly get approval for a mounting box which would allow a battery to be clipped in and removed. Approval might involve full stress calculations etc though and this only overcomes one of the many problems such as weight, space etc.
Rod1