So you put the phone in fastboot mode, connect it, run the RUU, and what happens?
Truth is I have a bad feeling that I know the answer: that wretched official bootloader unlock "upgrades" your hboot, and the RUU won't run because it finds a newer version already there (it's an "upgrade" utility). I'm not sure how well the old "downgrader" tricks work with this (I've never actually tried it).
You could try the following:
* find an RUU as close as possible to your phone's software version
* extract the ROM from the RUU into a .zip file, as described in this post
* unzip the ROM .zip
* fastboot flash the boot.img from the .zip file