you could stick a mod in, make up an evox boot disc and FTP a dashboard to the box,
slayers disc has a dashboard recovery function on it
you could carefully unsolder the eeprom from the 'broken' xbox and put it and the hard drive in an 'unbroken' xbox, that would allow you to prove if the one mobo is working or not, and possibly allow you to 'backup' the eeprom settings so you can copy them to your PC for safe keeping!,
you could also use evox to then unlock the hard drive so you can then re-swap eeproms and boot onto the 'broken' xbox, that would allow you to FTP to it and at least get evox running to start, then reinstall all the dash and stuff later on!
so, quite a few things to try :-)
as for errors, maybe the hard drive has been corrupted somehow!, or possibly error 16 means something else, rather than nulocked hard drive, the modding scene may have just associated that error with unlocked hard drive because thats when it appears most!