I actually had to drive a ways to go pick up Jasper when I was going to collect him because I had a hard time finding a Himalayan breeder in my area that I was comfortable with and that had available kittens when I was looking. The drive home was about 5 hours, and my friend and I stopped halfway to let him drink and see if he had to potty. He didn't go for the entire duration of the trip, but he did drink and eat a few kibbles. He was silent for most of the ride, but I'm pretty sure that's because he was terrified/tired looking back now. He hid under my bed for a while once we got back to my house before I got him to come out with a rope toy.
The second road trip we took is about 3 hr to my parent's house. I spent thanksgiving holiday with them, and Jasper rode in his carrier again. This time since he knew me better he had no qualms about voicing his displeasure with the situation. Letting him out to sit in my lap calmed him but I didn't want him running around the car and making a habit of that, so I kept him contained the entire trip back. He meowed sadly, then angrily, then bossily ... haha, it sounded like he was running the gamut of different emotional inflections to try and see what would provoke a response from me. He would periodically hiss and jam his face into the mesh like he was trying to force his way out
I felt really bad for him, but safety comes first, and ignoring him quelled his vocalizing/agitation after a while. Giving him any sort of attention seemed to restart it, so I found it was best to just ignore him altogether and check on him out of the corner of my eye. I used Feliway but I doubt it helped much.
Anyways, the ultimate point of all my rambling is that he was so agitated by being confined to the carrier in a moving car that removing him from it to use a litter box probably wouldn't have been feasible. I don't know much (Jasper is my first cat), but with him, I make sure he has time to use the box before we set forth just so I know he'll be OK on the drive. He's very good about using the box if you set him in it and he has to go, which helps. Common sense states that you should come prepared for an accident if she has one. I'm sure other, more experienced members of the forum can help you there. Otherwise, if she's anything like Jasper, and especially if she's made the trip before, I would guess that you'd probably best off doing what MowMow suggested and stocking up on snacks/drinks so you don't have to stop on the way.
Cheers and safe travels to you and Numly!
Jasper and I will be making the trek back to the parents' house for my university's winter holiday this Friday as well. Here's to hoping the kitties sleep, blissfully unaware of the scary road trip!