Sorry to add to the tide of disagreement, Muso, but I've been mucking around with my RY this evening and I get pinch harmonics all over the shop.
Is it the Zakk thing you're going for? I still can't nail that properly, but then I never could - I use the technique more on the treble strings, Billy Gibbons stylee. And for some reason it works better, for me, with upstrokes rather than downstrokes.