Not directly related, but as on the other topic we were discussing color backlight, which would need driving RGB leds with control of each channel, I searched quickly a solution to do that.

The TCA8424 has no pwm output, but 4 on/off outputs for leds. This allows 8 colors (all combination of RGB leds on and off) : black (=no light), blue, red, green, yellow, magenta, cyan, white.
The ADP5589 has one pwm output which is great to dim a one color backlight, but not enough to control the color. Combined with its outputs (if they are not used by the key matrix), we can have 8 colors with luminosity control.

If we want to fully follow the device ambiance or case color, then we can use a separate I2C chip to do it, as I2C allow having a lot of chips on the same bus.
For example the NxP PCA9633 (1,30€ only from RS) comes in a small SO8 package and provides 4 x 8 bits channels (Only 3 are needed here), so 24 bits colors are "theoretically" possible on the led. However leds intensity response from pwm is non linear, so this is not really true and has to be tuned, but a lot of colors are possible.
The good point is that the driver for this chip already exist in the kernel under the name "CONFIG_LEDS_PCA9633".
There are probably a lot of other chips that can do that, it was only the first one I found. There also are 16 channels version if someone wants to make a 5 colors backlight (like a color gradient) .

Then, to be able to have something smooth across the whole keyboard, several surface mounted RGB led (3?) can be connected in parrallel to the chip outputs (as a backlight doesn't need to produce a lot of light, and we want a low consumption). Some material that conduct light under the keys would help too.