You’re right about the withholding option! It doesn’t look like the report from CollegeBoard would say “x class’ AP score withheld”, but admissions offices could infer it anyway by looking at AP classes taken (assuming no self-study.)
However - Penn DOES have a score self-reporter in their admissions portal!!! It looks like I can just upload screenshots of my individual AP scores there under “other” (non-SAT-or-ACT)!
Thank you @tamagotchi!!!