Grab a copy of Ukelele and roll your own.

I would still need different hardware.

I remember there being a kind of "gaming" keyboard that let you move modules around for different configurations, but I think you were still limited to the basic staggered grid typewriter keyboard arrangement.

Most keyboards you can yank the key caps out of and rearrange them, but you still have that same problem of the staggered grid arrangement, and that many keyboards have the keys contoured differently depending on their row.

Then there is the Optimus Maximus keyboard with a tiny 48x48 pixel monitor in each key. It's been coming soon since 2005, but the guy who invented it is also a type designer so I'll cut him some slack.

But if you really want to move things around, perhaps the Ergodex DX1 Input System is for you. Still not the steampunk typecase you're thinking of, but you could easily prototype one with that...
Well you might need a few of them, since each only comes with 25 keys. It looks more useful for developing custom controller layouts.

I remember seeing this "keyboard" that's just light projected onto a flat surface, and you tap the surface to type. That should be pretty easy to reconfigure.

Put "keyboard" in engadget.com's search.

hhp

How about an upper and lower case keyboard:
http://www.synthmania.com/Famous%20Sounds/Images/KeithEmerson.jpg

According to the Optimus Maximus website, they have been shipping their keyboard since March of 2007. It's very pricey (just under $1700!), but if it works as advertised it would be just the ticket for people who work in multiple languages or who want lots of keyboard shortcuts for programs they use heavily. Take at look at the screenshot from their site showing a keyboard configured for Photoshop.