According to wikipedia, anti-accents: accents that tell you to play a note softer than the surrounding notes, which are indicated by a breve (slightly softer), parentheses (significantly softer) and brackets (much softer), are used in percussion music. But if you compose a piano piece, are you allowed to use these anti-accents? Are they just rare for piano pieces? Or are you not allowed to use them?
I've never seen them in piano music. Although I can't remember where (was it Prokofiev who liked to do it?), I have seen an "sp" written on a particular beat, to mean "subito piano." Perhaps you can use that. You could examine Beethoven's habit of writing "sf" over various notes for the opposite effect; I suspect that most pianists' familiarity with his music would make your intent pretty obvious.