I dont know how plausible this is. But could something like a 3D printer make custom keycaps?
I have seen pics of keycaps printed on a 3D printer and they were quite gross looking and I am sure the mount was unusable. Of course it might have been done on a "low end" printer. The printing is just very inaccurate. Please wait 20 more years. 
Well, I don't know about 20 years. I mean, just comparing cheap 3D printers from 5 years back to what we have now, it's not too terribly bad. I'm sure a low-end stereolithography machine will be made (if it hasn't already: Check kickstarter) that will provide better performance than these reprap-like ones. Maybe I'm just biased, but I really don't think these additive rapid prototyping machines are good enough, especially at the extreme low end.
Ideally (In my mind) we'd be able to get a decent SLS one (Where it sinters it together with a laser) or something, where we can fuse plastic granules directly, then pop it in a sintering oven for a few hours. I'm somewhat well up on powder processing, but that was mainly geared towards metal/ceramic. Maybe it's just the materials side of me, but I think ceramic keycaps would be awesome. Unfortunately, you need a lot more energy density to SLS ceramic (Like Alumina) than you need for black ABS, or other polymers.
Currently, the best method for direct keycap DIY seems to be casting. There's a lot of good information at the link precarious posted, above.
