Even if it's a moderatley good idea, what's the point?
The second you call dial the number, they start tracing the call. Even in these far fetched situations like choking where you have the ability to text, but not to forcibly dislodge the blockage from your throat or find someone else to OR hiding in a closet from a murderer, dialing 911 and dropping the phone on the floor is just as effective. They'll track the call and dispatch all 3 emergency services immediatley.
What's the point? If you can't speak, you just dial 911 anyway and they'll show up. You don't need to say anything on the phone, you just need to dial the number... texting just gives another way for people to crank call them.