I think it would be fine. The reason for not BD before a pap smear is because it can affect the sample of the cells they take from the cervix. Can't think of any reason why this would be a problem for the ultrasound. (Internal - a little icky but so what!!)

With the appearance of the EWCM and the ultrasound, you'll be able to see if a follicle is preparing itself to O! Ask them to check that too!

Good Luck.