Also, if the mentioned "key" used on each end is well-known or the xor approach is not cyptographic in quality, then anyone could listen to this EM output and c

Also, if the mentioned "key" used on each end is well-known or the xor approach is not cyptographic in quality, then anyone could listen to this EM output and compromise your "hard wired" data which you'd (forgivably) expect to be secure, a la TEMPEST. Depending on the strength of the signal, could be a serious problem.