60 days into her pregnancy you can use ivermectin which will both deworm and kill off any latched ticks that may have been missed or too hard to get.
As for stopping the ticks from latching on; Permethrin wipes are a good repellant. I would personally either purchase low content wipes or buy concentraited Permethrin and add a 10% Permethrin/water mix to a spray bottle and apply to her coat.
This way you can be sure that she won't have an adverse reaction to it as some products such as equi-spot contain 45%+ Permethrin which can cause skin reactions.
Other things to help keep ticks off her face could be applying a fly mask. It's not a guarantee but it will give them another barrier before reaching your mare's face.
I understand your reluctance to try different products when your mare is pregnant. However, please don't forget that ticks themselves can be very dangerous for both your mare and the unborn foal.
If in doubt, always ask your vet for another opinion!