You can go to [0] to get just text on a keycap with a fairly easy web-ui to describe the text, position, and size you want it.
Or you can take it entirely into your own hands and go to [1] to give them a vector file that they will custom print on to the keys.
They also offer that service in a "full keyboard set" option [2], so you don't need to do a ton of keys individually if you want to do the whole keyboard (although the full sets most likely won't fit your profile, but they do fit a good number of more standard keyboard layouts)
Again, they aren't the best quality, but they do a good job, and they take care of their customers. Plus the amount of customization you can do is pretty fucking insane. [3] is always my go-to example for just how customizable they are. Not only full color printing, but also individual key plastic color selection.
[0] http://www.wasdkeyboards.com/index.php/products/printed-keyc...
[1] http://www.wasdkeyboards.com/index.php/products/printed-keyc...
[2] http://www.wasdkeyboards.com/index.php/products/keycap-set/1...
[3] https://www.flickr.com/photos/wasdkeyboards/28965975050/
That or just dump the ergodox when you realize that it's just a pale sad imitation of a kinesis advantage.
The one you mentioned doesn’t even look pretty. ErgoDox EZ Shine is wonderful both in use and in how it looks!
I now have a button that can launch a terminal window, a button that launches HN, one that does a git pull on all my current projects, etc...
Like you said, it's a niche market, but it's one where most people are happy to pay the higher prices for something more tailored to their needs, and it's a rare "physical" area where a guy in his garage can compete with big companies.
I just recently got a new 3d printer, and I think after I get it more tuned I will be able to print fairly good quality keycaps. If they are good enough, I might try selling custom made-to-order keycaps if there is any interest.