Penguinmoon, what happens on ua.com when you search and click the radio button (at the very bottom of the page) for upgradeable flights (use miles to upgrade, or similar)? It should take you to a page that asks you to click the segments that you want to upgrade.
Then, on the results page you will see in red font, that certain flights cannot be upgraded or an upgrade is not available. For others, you won't see any red ink. That way, you can learn which ones have upgrade space available. And, the different routes will be displayed, so you can choose which connecting city you prefer.
It doesn't stop you from having to click on Fare Class for each segment once displayed, to make sure that the proper fare for a GPU or miles is available though.
I hope that helps and that I didn't miss your point entirely.
