If it's a harmonic, he could just be touching the string, instead of tapping a note 12frets up you can just lightly touch the string 12 frets higher and you'll get the harmonic to come out, if Slash is playing at the 19th fret, all it takes is a little experimentation to find the right spot to touch, just because their are no frets underneath it makes no difference, start hunting halfway between the bridge and the 19th fret and maybe you'll get lucky. Having said that, there is another possibility, Slash could just be muting some strings with his hand, and letting the amp feedback the sustained note, most tube amps will sound the harmonic when they start to feedback.........