Well, here's one way to push your customizable keyboard to the limit. Upholding the time-honored tradition of "if it has a display, we should play Doom on it", Redditor AyrA_ch posted the footage above to /r/videos. In it, they run Doom 2 on one of the tiny 48x48 pixel displays found in each key of the Optimus Maximus keyboard, which can be programmed to display pretty much anything you want.
Interestingly, AyrA_ch explained that the keyboard isn't actually displaying the game itself. Instead, they set it up so the main display the game is running on will constantly take screenshots. Those screenshots are quickly converted to the right format and size, and then played in order as a video on the key. The effect is basically the same, though I imagine there's a noticeable amount of lag. Nonetheless, it's a pretty brilliant workaround to solve a very silly problem.
If you are starting to get keyboard envy, I've got some bad news. Not many of these keyboards were made, and production has since been stopped. But even when they were available, they were extremely expensive at $1500. Incredibly cool, but not the most practical thing in the world—which makes sense given the company that made them is primarily an art and design studio, not a gaming peripheral manufacturer. Still, you can find the Optimus Maximus keyboard's official site here if you want more info.
You can also see a gif below that AyrA_ch posted to /r/gifs to show off what the customizable keys are usually supposed to be used for (memes, obviously) as well as getting a little context for just how tiny that Doom 2 screen is.