If, and that's a big if, the diaper is even seen, all that's going to happen will be one of two things. No reaction what-so-ever, or the person will laugh after you pull away and tell their coworkers. That's it.
Stop trying to force this fetish on other people. There's no need. This is where the line gets crossed.
They can also refuse to serve you there again and ban you from that drive through if it causes ENOUGH offence.
Compared to the real difficulties that people working a drive-through window can face, I doubt someone wearing diapers would even register. My inclination is toward discretion but it's not as though it's indecent exposure.
See what would happen if a man/women walked down the road in JUST a diaper though.
They would be arrested for indecent exposure and probably sectioned under the mental health act too.
Fully exposing your underwear in public other than the beach,swimming pool or gym changing room would count as indecent exposure.
Did any body participant in or see the Philly (Philadelphia, PA USA) Naked Bike Race a couple weeks ago?
No arrests were made and there were police around.