I think davie covered “Through the fire and the flames” without a pick once and the thing for me about using a pick is effective alternate plucking, which he did as to get up to speed for the song. So he must have used the index as well as the thumb.
I’ve seen a few players do that but very rarely and even Davie played it with a pick later on. So for the possibility of playing fast with alternate plucking a real pick may still be preferrable.
I think in your case where it’s about the sound created that is a viable technique if you can pull it off smoothly and as you said it has the benefit of switching between techniques as needed without having to throw something away or put in a pocket or something.