Don't think I would tranq in this case......she needs to be able to stand under the weight of the stallion........so needs to be fully alert.
I had a maiden mare who was acting like this for a couple of years. I came to the conclusion she was just too young and at 5 years old, she decided she was ok with the stallion and allowed him to breed. But she pushed it until I had actually called the vet and he was pulling up the driveway as the mare decided to stand and let the stallion cover her. So, turned out not to be necessary after all. Just suggestions.