Aha, worked it out-- seems you need to tell the server_default command if you're sending in some SQL that needs to be executed on the DBMS itself:
from sqlalchemy import text
class Test(db.Model):
id = db.Column(db.Integer, primary_key=True)
created = db.Column(db.DateTime, server_default=text('now()'))
That generates:
CREATE TABLE test (
id SERIAL NOT NULL,
created TIMESTAMP WITHOUT TIME ZONE DEFAULT now(),
PRIMARY KEY (id)
)