Some Swiss knives have ball pens. These pens are also sold separately (e.g. [1]) and work well as a stylus. At least mine doesn't actually write (with ink) on the display.
They sort of fit into the N900's stylus bay, but they are shorter (70mm) than the original stylus (100mm) and their end cap is smaller, so it's easy to get them in but hard to get them out again (the cap tends to get stuck).
The end cap is made of plastic, so it might be possible to glue or screw something on it to prevent the pen from vanishing in the stylus bay.

[1] https://www.swissknifeshop.com/shop/...oint-pen-large