Someone kindly emailed me to tell me that Braille watches are already available for the partially-sighted. These produce the time in a raised Braille pattern of dots when a button is pressed. So I've moved this to the "already done" list.
Ever wanted to know what the time is when stuck in some interminable face-to-face conversation? What you need is a glance-free watch, that enables you to tell the time without looking at it. Know how long you've been stuck without offending anyone!
This has to work by touch if the person boring you is not to realise that you are checking what the time is. For example, the watch might have a little solenoid that prods your wrist a number of times corresponding to the hour, then tens of minutes, and then minutes. Or vibrates in pulses, or even delivers small electric shocks. (That should stop the bore from sending you to sleep too.)
Some means of telling the watch that you want to know what the time is is also required, if it is not to do it every ten minutes regardless. A button would be tolerable, as the action of pressing it could be passed off as idle fidgetting, but better still would be some subtle and unique movement (a couple of quick rotations about the axis of the wrist, say) which could be done just with the watch arm. You get the idea.
Naturally, the watch would still have a conventional display of some kind, so you still have the option of looking at it obviously to send a blunt message to those bores you don't mind offending!