Add my own unsuccessful experience with using Star Alliance upgrade award on Eva.

Called in Eva to ask about star alliance upgrade award space availability for some flights that I want, and was insistently told to call United or ANA or whichever airline I'm wishing to apply the award, which is very confusing cause I'd assume Eva controls those availability. Eventually just gave up and asked about Z space and was told all available in my interested flights.
Booked and paid for the B fare on two flights SFO-TPE and all unable to be upgraded by neither United nor ANA complaining about no space left.

I guess the hardest part of this process is just looking for availability. Theoretically if you don't mind putting multiple $1000+ hold on your credit card then you can just book and see if you can upgrade but I just give up after 3 tries...