Resizing accidental in a chord

[Rodolfo Zitellini]

Dear All,
The house style of my publisher wants suggested accidentals to be
displayed usign a smaller font than "normal" ones. This is done easily
overriding the font size for every accidental. BUT how do I do this is
a chord? Let's suppose I have a three note chord

<fis ais cis>

And I want the cis to have the sharp smaller, how do I accomplish
this? If I do an \override, all the sharps will get smaller (because,
it I understand correctly, all the notes in the chord happen at the
same moment), and I can't use \tweak (the notation manual states quite
clearly that accidentals can not be modified with \tweak)
Any ideas?

Cheers,
Rodolfo