Ok, I'm in a very similar situation, although I know have purchased a mileage earning fare. I signed up for Asiana Club primarily so I could actually select seats after purchasing my reservation (which was made through Orbitz); I added my OZ number at booking and was successful at getting the seats I wanted.
Now I want to credit my flights to UA. The OZ call center suggested that adding my UA number to the reservation would prevent me from viewing it online. Is the best option to add my UA number when I actually check in? And will I be given the chance to do so?
This is my first time flying OZ, so I'd like to try to get any potential issues resolved early (the actual flights are in October).