As for the "Tone is in the hands" argument ... I believe this to be mostly true. I'm sure you've all probably seen this video:
Jerry is probably playing through some kind of pignose or small practice amp... clearly not his stage rig (I presume)... Anyway - he still has that unmistakable Jerry "calculatedly sloppy" feel that is so damn hard to emulate, and has nothing to do with sound processing and everything to do with his hands. Granted - the twangy sound of his guitar is a factor too, and like Vic and others have stated - everything adds up; guitar, strings, pick, amp, etc - it all adds to the sound.
But technique is technique. If you listen closely to nearly everything Jerry did, you can hear the difference when he is playing with a pick and when he's using his fingers - and you'll notice that he intermixes both quite frequently. I tend to do the same thing on a lot of tunes as well - I use my ring and middle fingers to pluck some strings while using my pick or thumb on others - all kind of intermixed together. I can get close to that "calculatedly sloppy" feel by doing this.
Anyone can learn to do this - just sit down and learn some straight up fingerstyle stuff (Garcia/Grisman, DJango stuff) just to get a good feel for using your ring and middle fingers on the B and G strings (and your pinky on high E)... then gradually teach yourself the chords to something like "Crazy Fingers" or "Lazy River Road" using fingers, pick, and thumb intermittently together... with some practice, you can get close to that Garcia feel - just by virtue of the inherent difficulty of actually using all that technique at once.