Working with my son right now on his dragon book. He has a lot of cool ideas and I like how the book is shaping up.
After going around a couple times on the system we wanted to use, we decided today that the book will be stated up for 1st Edition Advanced rules.
We felt there are a ton of books out there for 3e. We were not really interested in doing it for 4e though either. Basic was my original choice since it can be translated well. We even very, very briefly considered Savage Worlds (his new favorite).
I need to read over the OSRIC license IF I want to indicate compatibility with OSRIC (no idea if I want too or not yet). Of course there is the Labyrinth Lord "Advanced Edition Companion" and that is also a good candidate. They both represent the same era of play and are largely compatible with each other.
I think this is a good way to go to be honest. His ideas are very much n the mindset of the earliest days of the hobby; when everything was permitted and no one said you couldn't have a blue dragon that had ice cold fire as a breath weapon. HTBD will be a new point of view on these creatures, something I think we could all use after nearly 40 years.
Not sure on the deadline or when it will be out. But the author is working on it when it is his turn to use the homework computer.