The thing is that this could be seen as some sort of discrimination. I broached the case of town centres with poorly designed shared spaces where cars people and bikes shared the whole space with a company making gps for the blind and they said they would like to warn blind users about the dangers, but there were legal issues to it. I find this hard to believe myself. its probably more true to suggest that as no go areas, for whatever reason are often very changeable considerable research on a frequent basis would be needed to keep them up to date, and as I mooted above, what is no go depends very much on who you are and why the area is no go. Its better perhaps to allow users to input their own?
After all if you know an area has pitch battles of drunks at late night drinking holes, or you know during the school run times the traffic on certain roads is impossible I see no reason why this should not be entered and the sat nav could then avoid such places at such times. Local knowledge is often better than something entered by a company on national databases which could get that company sued by, say local businesses disputing what they put. Brian