I don't know if this message will reach too late, but here are my thoughts on your dilema: why don't you try the catch bag [or whatever it is called] first. Plan to give it a certain amount of time, preferably following plenty of fluids. Then, run water in the exam room or dip her hand into a cup of water to see if that will precipitate the needed action.
We took our son to the emergency room at one and a half because a serious case of gastroenteristis left him quite dehydrated [which breastfeeding was simply not addressing]. The ER wanted to test him for a UTI [since he was uncircumcised]. This was late at night. We'd been there for hours. They wanted to test him for everything. We REALLy didn't want him to be catheterized if we could avoid it. They "gave" us an hour, and at the final moment, with the bag still dry, my husband thought we should see if we could trigger his bladder. THankfully, it worked like a charm!
I'm not a medical professional, but I would offer my unsolicited, untrained advice to not wait long before your daughter is tested. I know that UTIs can have real health implications, and also, having suffered from them as an adult, know they are not pleasant.