Might be worth calling United; one possibility is that they simply don't have that city pair loaded in the computer. However, given that things like BZN-YUY are bookable on United, I'm really curious to know what city pair isn't possible!
If you make a booking with AC, and enter your UA number, it will automatically recognize your *G status, granting you things like priority boarding and luggage, and lounge access, but no, you can't use PlusPoints to upgrade on AC metal, just as I (regrettably) can't use eUps on United metal.