On the up stroke, I have to catch the string with the side of my index finger rather than my thumb on the downstroke. It's trickier than the downstroke, but is exactly the same in that when you get it, it's easy...

To be honest though, I don't find upstroke pinches very useful and don't use them when improvising - when you hit a harmonic you generally want to sustain it - they're not often used in fast runs. One tune I do uses a sucession of 4 of them, but I just play them all with downstrokes as I can hit it harder and squeeze more out of it!

I'm not sure how limiting it is to only be able to do them on the downstroke to be honest...