Custom error message for unknown when dump only

I'm just getting started with marshmallow so if there's a more elegant way to solve the issue please let me know.
I have the following simple model and schema.

class Instance(db.Model):
    __tablename__ = 'instances'

    id = db.Column(db.Integer, primary_key=True)
    hostname = db.Column(db.Unicode, index=True, unique=True, nullable=False)
    port = db.Column(db.Integer, nullable=False)

    def __repr__(self):
        return '<Instance: {}>'.format(self.hostname)

class InstanceSchema(Schema):
    id = fields.Integer(dump_only=True)
    hostname = fields.String(
        required=True,
        error_messages={'required': {'message': 'Hostname required', 'code': 400}}
    )
    port = fields.Integer(
        required=True,
        error_messages={'required': {'message': 'Port required', 'code': 400}}
    )
    @validates_schema
    def at_least_one(self, data):
        if self.partial and not len(data.keys()):
            raise ValidationError('Provide at least one valid key')

As you can see id is dump_only. If I try to load a simple dict (in the real application this would be data of a PATCH request)

try:
    result = InstanceSchema(partial=True).load({'id': 4})
    print(result)
except ValidationError as err:
    print(err.messages)

I'll get that id is an unknown field. So I have to manually check for it to provide a better error message like

    if "id" in err.messages:
        err.messages["id"] = "id is readonly"

It would be cool if it was possible to have a custom error message like hostname or port like

id = fields.Integer(dump_only=True,
    error_messages={'dump_only': {'message': 'id is readonly', 'code': 400}}
)

No file or test is named in the issue. Start by tracing how Schema fields.Integer(dump_only=True) produces the unknown-field ValidationError during load, then inspect the existing error_messages handling; done means a dump-only field can provide the requested custom message and the behavior is covered by a test.