Check out Unicomp keyboards: http://www.pckeyboard.com/page/category/UKBD
A common misconception. The USB HID Keyboard Boot Protocol, aka the lowest-common-denominator mode, is very limited and is where this belief comes from. 6KRO + modifiers, limited refresh rate, etc.
This mode is only required for compatibility with certain BIOSes and pre-USB operating systems, though some "gaming" keyboards start in this mode and only switch when they've been twiddled by their driver because a subset of those certain BIOSes won't even boot properly with a keyboard that goes beyond the boot protocol connected.
When booted to a remotely modern OS that has a full featured HID Keyboard driver it can support NKRO, up to 1000Hz refresh rates, etc. and as far as I'm aware is able to match or beat PS/2 in all ways.
Anytime I sit at a machine where the USB chip happens to reset, I wish it would use PS/2. It's just not the same, and I'm not even using n-key-rollover.
I can't type on a horrible staggered keyboard anymore. Once you get used to it (about a week for me), it just feels so natural. I can never go back. Staggered keys are perverse, all the contortions necessary just to locate the keys with your fingers.
The only problem is that now I look like a total retard when trying to use anyone else's keyboard, which is a pretty regular occurrence unfortunately as I'm in IT.
I'm pretty sure the surface-mount parts are already done for you on the Infinity Ergodox, switches are just about the easiest things to solder because they're quite large and the pins are far apart.
So yeah, shouldn't be too hard, and you'll have a sense of achievement when it's done! I love using a keyboard that I've assembled myself.