Look for any tutorial about animating a signature, then just use the position of the mask that reveals the writing as a motion path for an image of someone holding a pen with the anchor point of the layer set to the tip of the pen...
Yes, I experimented with that and it should work fine. Thanks for the suggestion.
Rick is completely correct, that's what he does around here. I will add, however, that emulating a writing instrument is not so simple. A pen is an extension of the hand and arm so it has organically mechanical modulations, pauses, changes in velocity and, especially important, lifts. You must account for the time the hand moves from the end of a word or sentence to the next task and to come back to dot the eyes and cross the tees and add punctuation. The first few times I tried this, back in ye olden days of AE2+, we videotaped someone (with a camera and videotape) using a fountain pen and then tried to match the keyframes of the mask to simulate the action and movement of the pen's nib in her hands.
Before AE, we shot the underside of a glass panel on which someone wrote with a broad nib filled with India ink. This was used as the inverted key source for a text reveal.