Just for the hell of it, I decided to try SGI granite keycaps on my Apple Extended Keyboard. Here's how it looks:
There are a bunch of keys that don't fit. All the keys on the top row are not exchangeable because the switch is oriented vertically instead of horizontal (horizontal on the SGI). The Caps Lock switch is slightly more to the right on the SGI, so it can't be used or it will snag on the A. Numeric keypad Enter and 0 can't be used because the stabilizer bars are on opposite sides from the ones on the SGI. The SGI has one large + instead of two smaller keys for + and -.
(I just noticed I have grey * / and - wrong, so I switched them. The - is actually an =, but there's no grey = key.)
Maybe Control can be switched with CTRL, but obviously there is no substitute for the option and Apple keys.
I like the font on the SGI keys better, but it looks a bit odd having two different fonts so I might switch it back. What do you think?