In a safe protected by a pack of ravenous IP lawyers.
Be interested to hear how you get on.
Yeh - I just emailed steinway to ask if they'd sell me one of their the cast iron frames if it was unbranded hahah - I'm not holding out a lot of hope, but will have to push this approach with as many makers as possible as I don't think i'll get a quality one if I just try to design it and take that to a cast iron factory.

its obviously going to be a very long term project, and at this stage its really just seeing if it will even be possible, I like the idea of trying to produce something similar to a steinway 'O' - its probably a little unrealistic though

I did find these guys - fandrichpiano.com - they consult on piano design with actual manufacturers so I may be able to get some ideas out of them, and some more comprehensive specifications than the "this long, this heavy" you get from well known manufacturers. I emailed them, we'll have to see if the cost is prohibitive.
Also, have you broached the subject with your fiance? 
Yes, not an issue - I knew it wouldnt be. She's a professional dog trainer, which means I often endure and help with dogs that have severe behavioral problems (I've been bitten several times) - consequently I have the moral highground, and she has to let me do the odd thing that she mightn't like... not that I mind the dogs, and I don't think she'd really mind the piano since I'd make sure it didnt really effect the rest of our lives
too much