Ah, I've been trying to hold onto this secret for as long as I can but I can hold it no longer...
I'm not sure how Joe does it but I'll tell you how I get harmonics on the upper frets and that is by using
upstrokes. You have to slightly change the angle of your thumb and finger so that your thumb looks like it's almost bending inward, not outward and your index finger should be facing away from you
Do you remember how we were all taught NOT to hold our pick between the back of our finger and the thumb ? Well, that's kind of what you have to do for this technique. When you do the upstroke you have to catch the string with your fingertip as well (or is it thumb, I'm describing this from memory) but it feels as if you really are pinching the string with an upstroke.
The other important factor is that you have to pick in the correct place according to which fret you are on. Around the 15th, you'll be closer to the neck.. any higher and you start moving back toward the bridge, but it's only in small movements. Imagine that the frets continue under the strings that you are picking and it will help you gain an idea of how small the changes in direction need to be to get the harmonic. You will generally be picking in the area over the neck pickup (if you have one)
I hope that helps !