Hi Tabby --
While I admire your inspiration to help others with CFIDS, I have a bit different take on what you suggest. What you propose adds more complexity to someone's life in having to learn how to use a device, not to mention having the constant reminder of limitations on one's wrist. Some sufferers are so overcome that they might have difficulty remembering to use such a device regularly for it to have any possible benefit.
And how would such a thing be programmed, with many individual threshold fatigue points amongst individuals? It would be impossible to create a device tailor-made for each person. And even within individual cases, fatigue is not reached at the same point each day...all things being equal.
I think it's critically important not to "embrace" ME/CFIDS and make it first and foremost in one's thoughts each and every day. Yes, it is there, but it's much more important to focus on living than limitation. What you suggest is somewhat like the proverbial flea in a glass jar with a lid. The flea learns not to jump against the lid to try to escape, and once the lid is removed, the flea has been trained not to jump any longer and remains in the jar. (Probably not what would really happen, but the illustration is helpful.)
Many people with ME/CFIDS have been helped with LDN. I think that's a much more positive way to manage the condition, both as a practical matter, and also in terms of one's mental health.
I don't mean to dampen your enthusiasm -- but there might be a direction you could take your thought to help others that could reach more folks.