I doubt you can get arrested for that, the most you'll get is some weird looks, but honestly people shouldn't be looking in your car to begin with (Unless you get pulled over for something then thats kinda a cop doing their job to make sure you don't have a weapon) But even so, around here I've seen people drive around with just their boxers and a tanktop or bikinis (Which the ladder certainly shows a lot more than a diaper would) If you get pulled over and a cop says anything, its an easy excuse to get out of it, your pants got wet/ripped/soiled somehow and it made your driving uncomfortable and you didn't have a spare pair of pants, so you're driving in just your underwear. The cop would most likely rather you drive comfortably than in an uncomfortable/dangerous situation (Like say if you spilled hot coffee on your pants) And even so, if they take your to court say the same thing to the judge, that your pants got messed up some how and you didn't have a spare pair because well, who carries a spare pair of pants around?
Edit*: And if you're erm, soiled/wet you can probably get away with changing in your vehicle (SUVs with tinted windows are probably recommended over a VW Golf, lol.) And they can't really say anything to you because well you're not doing it in public and if someone tries to say something say its uncomfortable to drive like that and that they should mind their own business.