Just like with Pierre Schaeffer’s cut bell, when you remove the plucked attack from a guitar sound, you’re removing the most obvious perceptual anchor that tells you you’re listening to a guitar. I really think that’s the best way to “process” a guitar to sound like something else. I like to either edit/trim away attacks (if I’m working with samples) or find other pickless/strumless ways of playing. Or both. Reliable favorites: knocking/tapping on the body without touching the strings, metal rods or chopsticks dulcimer style, a screwdriver as a combination bow and slide, paintbrushes and string (there’s nice bowing in that video too, naturally).
A nice thing about lap steel guitars is that not only is there lots of room for preparations (springs, alligator clips…), but you can play behind the steel the way you might play behind the bridge on a Jazzmaster or other floating-bridge guitar – this sounds especially good at the 7th/12th/15th/etc. frets (anywhere with a nice harmonic ratio between the left & right parts of the string as divided by the steel).
Wind is magical, as is @bassling’s video. Equally hands-off but a bit less poetic are transducers or bass shakers, which you can use to “play” a guitar using sound (attach to the guitar, drive with a power amp, play whatever you like into the amp). Before I got into transducers, a friend and I once tried to build a Fernandes Sustainer-like “anti-pickup” that could be fed with audio and vibrate strings directly, but we either had way too many turns of wire or way too few, or something. It didn’t drive the strings very well, and we were only able to play Yes’s “Heart of the Sunrise” into it a couple times before it started melting.
Now that I think of it, other than the low bass throb and the synth strings toward the end, I’m pretty sure the second half of this set was all guitar samples, with very little processing other than delay and changing playback rate: https://soundcloud.com/synthetivvision/wet-colors-20180722
