How to report errors on non-column input fields - cakephp

Question

I have a form for creating a batch of events. It's got inputs for name, description, location, start and end time, all of which are columns in the events table. It's also got a number of inputs for dates, using field names like dates.0.date, dates.1.date, etc.
echo $this->Form->input('name');
echo $this->Form->input('description');
echo $this->Form->input('location');
echo $this->Form->input('start');
echo $this->Form->input('end');
for ($i = 0; $i < $this->request->data['repeat_count']; ++ $i) {
echo $this->Form->input("dates.$i.date", ['type' => 'date']);
}
The save process loops through the dates array in the submitted input, creating an entity with the rest of the data and the date for each one, and saving it. That is, the resulting entities will have fields called name, description, location, start, end and date. (Hope that's clear!) This part works great, and it's wrapped in a transactional function so that the events are only saved if all succeed.
$event = $this->Events->newEntity(array_merge(
$this->request->data, ['team_id' => $id, 'dates' => []]));
if (!$this->Events->connection()->transactional(function () use ($id, $event) {
for ($i = 0; $i < $this->request->data['repeat_count']; ++ $i) {
// Use $e for the entity that is actually saved, so the
// $event entity isn't mangled
$e = $this->Events->newEntity(array_merge(
$this->request->data,
[
'team_id' => $id,
'date' => $this->request->data['dates'][$i]['date']
]
));
$this->Events->save($e);
}
if ($event->errors()) {
$this->Flash->warning(__('The event could not be saved. Please correct the errors below and try again.'));
return false;
}
return false;
})) {
$this->set(compact('event'));
return;
}
The problem is when an invalid date is provided. The entity is not saved, and the error is correctly located in $e->errors('date'). But I can't figure out how to report that error back to the user on a date-by-date basis. Extrapolating from the entity documentation and using the dot notation that is standard elsewhere, I've tried $event->errors("dates.$i.date", $e->errors('date'));, and confirmed that any such errors are preserved in the list of errors in the entity when the view is rendered (i.e. if I use debug($event->errors());). But when I create the input fields, there are no error messages for the bad dates. They all have the submitted values in them, just no errors.
Tried using "date" instead of "dates" for the base of the field names (e.g. date.0.date instead of dates.0.date), to see if it was skipping the error check due to "dates" not being a column in the table, but this just caused a validation error at the point where I validate the rest of the data, so I didn't pursue that option any further.
I have some code elsewhere that sets the errors on associated entities which, if translated to this situation, would look like $event->dates[0]->errors($e->errors('date'));, but $event isn't associated with something called dates, so I don't see how to make this work here. I tried creating dates as an array in the $event entity, but assigning $event['dates'][$i] = $this->Events->newEntity(...); results in a PHP warning: "Indirect modification of overloaded element has no effect", and the entity is thrown away. Maybe manually create some ArrayContext object and assign the errors into that? Not immediately obvious to me how to do that.