would the following work? I remember doing something similar with the original evo 4g when 2.10 (or whatever version) hboot was "unrootable"
Do a soft root. then install (extract the zip file) flashimage. use it to flash a recovery and then use the recovery to flash a the eng hboot. or use it to flash both.
Like I said it worked for me before but it has been a while. Did they make the bootloader even more secure in the 3d?
Hope that this helps.