You can flash a ROM from HBOOT but it needs to be in the correct format. It may only work with the stock ruu. May not work with other custom roms. I'm not positive on this though.
Go to the Inspire forum on xda. There are a dozen postings from people with similar issues. There's even a guide for how to recover if stuck in HBOOT. Take the time to read through those and you'll find how to fix it. If you create a new posting over there with your problem, chances are you'll get a lot of grief from people about not searching for the answer yourself.