Thanks for all your inputs. We still have time until our next Japan trip so we'll give it some thoughts. We might try AZA's prem econ once and see if it'd be money well spent. The other option of course is to buy an upgrade-eligible UA economy fare and use our miles to upgrade, which we haven't tried yet... With our lowly elite status (2P) and the fact that we usually fly during the busiest times (like Christmas or New Year's) we're concerned that the upgrade would not clear even after spending extra few hundred dollars on upgrade-eligible fares.
I do realize that certain Y fares on ANA would only give me 70% of UA miles, but that's not a major concern for me--we hardly ever use our miles anyway, so we're not all that into accumulating our miles. We just want to fly a little more comfortably...
Anyway, many thanks for all your replies!