Booking Y/B on AC is not cost effective in my experience. AC charges nearly as much, sometimes more, for flexible fares as it does for business class.

If you want to use *A Upgrades I think your options are LX or SAS which allow upgrades from cheaper fares. For LX I would only book if there was I availability at the time. For SAS you could book Premium Economy and if the upgrade never clears you at least get Premium Economy. But that has the same problems as optiontown.

Otherwise the option of buying US Airways miles is good.

The other option is to look for UA SWUs or upgrades that people are willing to trade online. There are a lot of people selling UA SWUs online. Other programs usually dislike the existence of this market and have put barriers in place to reduce it.