I do not doubt it is a JTAG chip for all boards (they probably just vary where you throw wires) but the trouble is that the CPU has efuses in it which can be burned by software- with enough you can make a number that can be read and as such disallow old software* (why we can not JTAG every system).
*physically you can downgrade all day long and nothing would happen, as soon as you try to boot though it would lock up when it detects the old version running.
To this end we need a new exploit (unlikely- the security model means it can only come from a limited amount of places and MS has done a reasonable job at locking them down), a way to bypass efuses (what people are largely focussing on with low voltage glitching and superclocking), the crypto to be broken (given it is stuff used worldwide every day but many different things to the 360 this is extremely doubtful) or for the keys to be leaked/divined (not likely- MS knows well what would happen then and probably locks them down tight)