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?
There are anti-accents in piano-pieces by Schönberg and Kurtag. E.g.:
- Schönberg - Klavierstücke op.23, #5, Walzer, bar 68
- Kurtag - Jatekok
Anti-accents (or 'ghost notes') are not commonly used in piano music. So, of course you're 'allowed' to use them, but it would be sensible to add an explanatory note.
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.