Home Assistant’s Local Calendar complies with the iCalendar standard. It’s unclear to me if it supports custom fields.
Currently, you can use Summary to pass a JSON payload like:
{ "temperature": 20.5, "humidity": 42 }
Given an automation with a Calendar Trigger, you can reference the contents of Summary like this:
{% set x = trigger.calendar_event.summary | from_json %}
{{ x.temperature }}, {{ x.humidity }}