I suspect a backlit keyboard won't be viable.
Right now, all keyboards start off with the same set of blank keys, and then the labels are ‘etched’ with a laser (causing the plastic to lighten and expand slightly). All keyboard layouts can be etched with the same machine; it's very quick.
However, if the keyboard were backlit, each key would have to be moulded from two types of plastic (the opaque dark grey, and a translucent white), customised for the particular label. I suspect this couldn't be done in-house, but would need each keyboard variant to be carefully designed, ordered in, and stored.
Having different keyboard layouts probably complicates Planet's logistics enough already: the number of models they have to deal with is at least twice the number of keyboard layouts (one for Wifi-only and one for 4G variants — and more due to the different mains adapters). But ordering in keys with labels would probably complicate it an awful lot more. This is feasible for a company with the size (and margins) of Apple or Dell, but for Planet I fear it may not be.
And all that's before you consider the difficulties of piping light (whether through fibres or air) through the rubber membrane, the extra battery drain…
But don't get too downcast. I've been using that keyboard in bed almost every night for 20 years (first on a Psion 5/5mx, and now on a Gemini), and I've hardly ever had any trouble hitting the right keys. (There aren't quite as many as on a full-size computer keyboard, for one thing; and I find my thumbs learned very quickly where to go.) So I don't think the lack of a backlight justifies that much concern, really.
For me, things like allowing earphone buttons to work with the lid closed, getting the built-in speakers to work in-phase and in stereo, fixing the wobbly keys, ensuring the hinge is strong enough, and adding rubber feet are much more important.
