While a nice idea I am not sure how practical it would be to implement.
Three is primarily a Mobile phone operator, with home broadband still a minor part of the overall business (AFAIK) and even a wireless broadband hub may not be in the same postcode as the biling address.
So the first problem is how do they know which devices are in the same area as the mast where work is being carried out.
Secondly engineers carrying out work will do their best to minimise the impact to customers so you might get bombarded with emails warning of work that has no practical impact on you.
In all of the years that I had a wired broadband and phone connection I was Never pre-warned of any impact to service.
