Doesn't flashing a new software version rewrite the radio anyways? When I flashed 5.7.894, I had to re-do the radio hack to get it to work again. So even if you borked the radio hack, couldn't you hypothetically flash 5.7.894 or 5.6.890 (depending upon which you came from) and rewrite the radio's decimal keys?
Anyways, at the moment, it's nearly impossible to kill this phone unless you mess up the radio hack. Other than that, with ROMs and themes and what not, you can always return it to stock and have a working phone in under 15 minutes.
Maybe if I'm feeling ballsy some day, I'll copy and paste my 8040 decimal code into a Word doc and save it as a backup. Then I'll go in and change the codes for my 8040-8043 signatures and then try to put them back with the backed up strings or by flashing over a different software version. And if I can't fix it, I'll restore it to stock look and take it back. Making it my 2nd return and 1 return closer to getting a GNex.