I am not quite sure I'd be ready to recommend it, but your more adventurous patrons may want to experiment. These keycaps are PBT, a cousin of polyester. They are not particularly pleasant smelling when heated or especially when burned, but they're not as unhealthy as ABS (the other common plastic for keycaps) and certainly not as bad as the straight up poison gas that comes from PVC. I use a basic 5W blue diode laser, coat the keycap with an "infusible ink" pen from Cricut (most of their infusible products are polyester-based), put it in an alignment jig, then laser a raster image "low and slow." My particular laser seems to do best when I do two or three passes at 2% power and 45mm/minute. The idea is to heat it roughly in line with the crafting heat presses without letting the heat spread and color in areas beyond the beam. I experimented with actually burning or engraving, and that sort of works, but (1) it's stinky, and (2) the ash wipes away and you're left with a mostly colorless letter-shaped indentation. The "dye sub" technique produces barely any fumes at all. There are a few people on youtube who've tried similar techniques, and quite a few who have used different heat or dye sources.

Aesthetically, the process was only marginally successful, though I'm optimistic about the longevity, at least compared to other low-end manufacturing techniques. I've been using a similar set of home-lasered keycaps for about a month with little to no wear. My jig was not as good on that set, AND I tried to center the keycap legends, meaning every fraction of a millimeter was painfully obvious. These legends didn't end up exactly where I might have liked either, but they're all off by the exact amount (about 1mm), so being consistent, the alignment isn't too bad.