If it really is the reg, the simple option is to find some other source of 3.3V and supply it to the 3.3V PCB pin (makes sure it's not even slightly more than 3.3V though or whole board is toast). You may have to remove the regulator first if it's output is damaged in some way. Would be odd, since normally the regulators get hot and drop the output rather than failing outright. Does anything get hot on the board as it now is? Other possbility is some form of failure that pulls the 3.3v regulator output down, which would need to be fixed before trying external power.