Thanks every one for your input.

My experience is similar to what PaclantFlyer described. Even though the KVS tool, and united.com both showed no business award availability, I called EVA and asked specifically for upgrades. The representative at EVA told me seats were available. I booked the ticket on EVA.com then got immediate confirmation of the upgrade at United.com. It was rather easy and painless.

At least based on my experience, there seems to be different inventory for mileage awards and upgrade awards.