That is singularly interesting...I imagine u'd need custom work done, have someone modify a timepiece for u. The ash could be mixed with paints and some kind of fixer maybe, and applied to the watch dial.
One possibility that comes to mind are Seiko mods. The seiko mod community is quite extensive and are accustomed to commissioning work involving one-offs such as this, u just need to find a reputable modder who can execute the work. As far as watches go, Seikos are fairly ubiquitous with plenty of parts readily available for modification, maintenance, so ur watch will be well and running with spares aplenty for years to come. On top of that, even if in the unlikely even something does happen to the watch, say the case gets severely damage or something, u could salvage the dial to transplant into another seiko easily enough.
