No, Schnorr does not.. you can take a hash of your clear text and sign that though! That should be sufficient for timestamping. Sign a tamper-proof hash of the plaintext!
Discussion
The use case requires a "noob" to be able to manually review the content of a signed message visually and validating the signature by copy/pasting the clear text signature. Adding a hashing step sounds trivial but makes it far less user friendly
Visual verification like that would indeed be easier with PGP. One more extra step for noobs, and you’ve lost em. I understand!