There was a half-Colemak script out quite a while ago. From my experience with it, I wouldn't use the Space bar as its' modifier. The reason for that is that the modifier key may cause some confusion when hit incorrectly, and the Space bar is the most frequently hit key on the entire keyboard (about 20% of keystrokes)!

If you have a 105-key board you might use the middle key on the bottom row (I've put my VK_102 key there). Or the traditional VK_102 position. If that's not an option, maybe the CapsLock key might work out - although it could be a longish stretch to some keys (especially when using modifiers!) and it'd only work for left-handers. Then there's the Win/GUI key(s); you might use one and keep the other one intact for when you need to press a 'Win'-type hotkey. Left/right handed users could then choose which of them to tweak.

But the real coolness is in my opinion to use a foot switch. Either make one that produces a scan code that the keyboard doesn't, or do a solder hack on a keyboard you're not afraid of ruining. You might use the ScrollLock key for instance; the Break and SysRq keys are weird as hell scancodewise. You wouldn't need to send the ScrollLock key on release then, but would be better off sending that as a combo of pedal plus something else.

Tip: If you operate on scan codes instead of letters, you can make it layout invariant. It'd mirror your board regardless of what layout you were using.

I'm in favour of getting this into the Portable Keyboard Layout program at some point. You could do it today using the Extend functionality, but I'd like to have multiple 'Extend' keys to make something like this really useful.

I've been thinking more about mirrored keyboarding, as I think it's an awesome concept.

I think one selling point for Colemak in this respect is the way it prioritizes rolls! Rolls should be readily doable in a mirrored setup, whereas the otherwise nice enough hand alterations that for instance Dvorak prioritize may easily lead to same-finger situations - and those are just about the worst thing that can bother your typing.

Yes, my half Colemak works. The Linux implementation is ready out-of-the-box and the PKL one isn't hard to pull off (but at the expense of the Extend mappings). Check out my sig topics for more info and images and stuff, and ask if anything is unclear. :)