In his tapping pieces, Mongrain doesn't sound like he's playing guitar at all, but rather some other, harpischord-like instrument. Of course his effect chain is in part responsible for that, but his polyphonic playing style is IMO more important at that.
Regarding the (lack of) structure, it's worth knowin that both tapping pieces were written as an improvisation framework while he was making a living as a street musician in Europe.
It sounds to me like he is making a lot of use of harmonics. He appears to be doing it percussively (by banging his finger against the fretboard vs. the usual technique of lightly touching the string and picking elsewhere). This is probably what gives him his unique sound.
Regarding the (lack of) structure, it's worth knowin that both tapping pieces were written as an improvisation framework while he was making a living as a street musician in Europe.