Alright, it's been a few days, but I have some observations.
I listened to the first batch again. The percussion seems to give the guitar more of the piano's ability to be an expressive solo instrument. The percussion in these songs seems to evoke a dance in some and just some rhythmic interest in others (like "Aerial Boundaries").
Michael Hedge's "Rootwich" is based on a catchy bass line. The interplay between the bass line, the slapping on the body of the guitar and the chord strumming create a constant 16th note feel, make it feel a bit "funky."
The finger-picking in "Ragamuffin" is fantastic and very melodic. He doesn't bring in the percussion element until later in the song, when he introduces slapping. It has a similar feeling of a rock band arrangement when the drums come in part way through a ballad.
In some of these selections, I've noticed that Hedges often plays 2 and 4 on the guitar (the snare hits) and Preston Reed plays the 1 and 3 (bass hits). I'm sure they both do both all the time, I just wanted to mention it.
The Rodrigo y Gabriela piece is crazy. I've heard they kind of invented their own flamenco-esque style of playing. Gabriela's slaps on the strings and on the body of the guitar give a consistent feel that reminds me of clave somewhat, but I'm not sure exactly what it is.
Tommy Emmanuel was the first on this list to use a pick at all. His flat picking speed and consistency seems to keep the feel going even when he's not using percussion (which there isn't much of in this track). When he adds percussion, it seems to be for emphasis more than anything else.