Well the only sure fire way to see if its exploitable is to dump the nand and check the cb.
Xenon: 1921 or lower is Exploitable (exception: 8192 IS EXPLOITABLE)
Zephyr: 4558 or lower is Exploitable (exception: 4580 IS EXPLOITABLE this needs falcon version of Free60)
Falcon: 5770 or lower is Exploitable
Jasper 16mb: 6712 or lower is Exploitable
Jasper Arcade (256/512): 6723 or lower is Exploitable
It doesnt matter which version 360 you have AS LONG as the cb is one of the above and pre 8*** dash. I dont think the newer xbox yoiu have will work as you were right in saying that newer xboxs had the bootloader updated in them.
You might get lucky but the only way to know is to dump the nad and check the cb. Hope this helps.
That was very detailed and extremely helpful, thanks.