Hi Martin,
You could inject custom layoutXml and fetchXml to the Event registration field by executing the custom javascript function at OnLoad event of quick create form of Check-in.

Sample code:
function customView(executionContext) {
var eventId = new URLSearchParams(location.href).get('id');
var formContext = executionContext.getFormContext();
var fetchXml = ''
''
''
''
''
''
''
''
''
''
''
''
''
'';
var layoutXml = ''
""
""
""
""
""
"";
formContext.getControl("msevtmgt_registrationid").addCustomView('6db26390-685a-4d19-a274-aeaa771f1b5c', 'msevtmgt_eventregistration', 'Clofly View', fetchXml, layoutXml, true);
}
It will inject a custom view with custom layout(custom columns) to the event registration field and set it as the default view.
We can add columns we want to the layoutxml cell node, but they should have been queried from fetchxml.
(Columns I set: Registration ID, contact name and createdon date.)

To only show associated records of the current event, we use following syntax to get id of the current event.
var eventId = new URLSearchParams(location.href).get('id');
The custom injected view will be available in the list.(It only exists on the quick create form and it will not affect the default "hardcoded" view.)

Please refer to following articles to learn about addCustomView function and its example.
https://docs.microsoft.com/en-us/powerapps/developer/model-driven-apps/clientapi/reference/controls/addcustomview
https://www.c-sharpcorner.com/blogs/filtering-lookup-based-on-the-link-entity
Regards,
Clofly