For historical reference, this method was allegedly taught to the British SOE during WWII. It was also recommended for use with the Sidewinder SMG--an unusual SMG that braced against the bicep and could only be fired from below the line of sight (seen here on the cover of an old SOF magazine
https://www.nsfsmagazine.com/secure/sofstore/covers/1975-1979/1979%20October.pdf).
From a biomechanical standpoint, it's important to understand that the middle, ring, and little fingers of the hand are all powered by the same muscle group. That's where the real strength of the grip lies. The index finger and thumb are separate, which is why we can naturally grip a pistol firmly, yet still have the ability to operate triggers and thumb safeties independently without compromising our grip. Operating the trigger with the middle finger will compromise your grip and reduce recoil management.
And, yes, sticking angle iron to the frame of your gun is ridiculous.
Stay safe,
Mike