I had a similar fiasco with Avianca (*A) last month. Three observations:

1. I got myself a *G status by calling UA and begging them to front me the status, as I was a few PQPs short at the time. Since I was only a day or so away from reaching the *G status and they could tell from the computer, 40 minutes into the internal transfers I finally got it. The reason to expedite it was to get access to the Contract lounge at JFK on the way to UIO. That lounge is a joke (as noted by a few people on other forums), and it was closed as of 9 pm. I had a 1 am flight, so the whole idea was a moot point
2. My experience with luggage allowance was inconsistent on Avianca. Since I already got a *G, I was expecting free carry-on and a free checked bag. Which was the case if you looked at my boarding pass. It worked flawlessly on the way to UIO from JFK, and even on the domestic GYE-UIO flight within Ecuador. But on the way back in the Quito airport, the kiosk agent was less than happy. She said that my boarding pass is wrong to give me all that, since I had a XS ticket (equivalent of BE at Avianca). Which is somewhat true if you read the website. But I was pressing with her hard since I had no intention of paying $90, and was referring to my boarding pass. In the end, she relented, but only because I had time to spare. If I was in a bind and in a rush, I may have given up. So a word of caution to those who are trying to balance out the published XS fare rules (no freebies supposedly irrespective of status) vs the reality that the boarding passes happily throw this at you
3. The worst part was getting PQPs and miles on my UA mileagePlus account upon arrival. I have given it a few days and checked the UA home screen to satisfy myself that the *A partner miles posted. They did for the flight to UIO from JFK, but not the way back, which is coincidentally the flight that I had trouble bringing my bag on. For my GF, it was the other way around. A call into UA revealed that Avianca attributed one of the two flights to their loyalty program (which I never signed up for or cared to even research. since I had never flown them and never will again). But I did notice that one of my boarding passes had Avianca's miles account, but the other two flights had UA. Didn't think twice back then (other than muse briefly that this might be the reason they want me to pay for the carry-on), but made sense afterwards. It took me three hours with UA's customer service to get the miles credited to my UA MP. I ended up emailing them the original ticket I purchased months ago that had my UA MP account and no Avianca loyalty. To this day, I have no clue why Avianca assigned me their membership and replaced it on one of the boarding passes.

To ynwa28: I too tried to change my Avianca's loyalty number on the boarding pass back from UIO to JFK, to show my UA MP instead. And I too had not had any success. Which translated eventually into the carry-on argument, as well as no miles posted to my UA MP. So in the future I will be super careful to make sure that I only use my UA loyalty program and not allow any airline squeeze in theirs.