I just did request a star alliance upgrade on an EVA air flight. It went fine over their interface on the inbound flight, on the outbound flight they seemingly don't have availability, so I only got a very cryptic error message.

I agree it could be done much easier as it is really complicated to fill in the very detail itinerary in the form, but eventually I got it confirmed. I found it very good value for my particular case: 4.700 CNY "B" economy class leg upgraded straight into business for 30.000 points (13 hour long flight).