The usual way of doing backlight is by having the LED underneath the switch and letting the switch transmit the light upwards. Many switches have special cutout positions for this, so the lighting for all the square keys is identical.
The only issue is that the keycap itself needs to be partially translucent and partially opaque, so rather than laser-etching or printing a legend they need to be "double-shot"[0], which means a two-phase injection molding process. This makes altering of the keycaps quite expensive as you need a new set of molds - which is especially bad if you were using one mold to produce an entire set of keycaps at once.
It's definitely doable to ship it with two different number rows, but it's the kind of thing you really want to know at the very start.
