Re: Sustain as a function of sheer volume...
The pure sustain of a guitar isn't related to an amp. I know it sounds like I'm being a pecker, and I know what you mean about the interaction of a guitar and amp, but a guitar that sustains exceptionally well is a thing unto itself, and is a function of guitar quality. An amp , pickups, and volume may help that some, but theres no substitute for a great guitar that sustains well. This is not a function of resonance, although it can be. Some solid Electrics that have almost no resonance or volume unplugged sustain like mad, because they are high quality instruments. Nothing kills "natural" guitar sustain like a cheap hollow sounding , overly "resonant"( if thats what you want to call it, and most do) plinky and thin sounding budget guitar.
I will say this; Throw ten pounds of wood on a guitar, no matter what ot is, and it will tend to sustain!
In other words, if you think a good guitar sustains well with volume, a great guitar will blow you away with without so much volume, even with moderate weight.
FWIW I don't consider amplifier compression or feedback in and of itself "sustain". Almost any guitar will get that.