Hey lamb...not a whole lot you can do other than give the dentist a heads up that you may need breaks in the appt. The only time I have ever resorted to adult diapers was one dentist appt. Ihave a horrible gag reflex which sometimes leads to an oops....so just to be less stressed...I wore a depends for my crown fitting. I have considered it for other appointments becuase even having my teeth flossed makes me gag horribly.
The only other thing I can recommend for the future is make your appts. later in the day when possible. It's hard for my work schedule, but I gag less in the afternoon. I also found that I had less sprint to the bathroom worries as I get further into the day so maybe that would help you.
Hope all goes well and your pearly whites glisten!