In this fanfic, Percy could be wearing a dress because he has been magically transformed into a female form for a short period. He has to navigate the world in this new appearance. He might learn a great deal about the challenges and privileges that women face. The dress then becomes a symbol of his new - found understanding and empathy.
One possible plot could be that Percy decides to wear a dress for a special occasion like a masquerade ball. In the fanfic, his friends might be initially shocked but then come to accept and even admire his bold fashion choice. It could also involve Percy exploring his own sense of identity and self - expression through wearing the dress.