New CalendarEntity

Hi !
I’m developing a small 3rd party integration that is pretty specific (it is only useful to those who have 2nd grade schoolers in public schools of 2 fairly small administrative areas in France).
A previous implementation used the CalendarDeviceEntity and when I recently recoded it from scratch I used the new CalendarEntity (as per this blog post). This entity is used to let HA know about class times (and particularly class ending times) for automations, but also to display classes with Atomic calendar revive.
Now, before 2022.5, I was (sort of) emulating the way Google Calendar data looks in that integration to mark cancelled classes as “declined” with some json attributes that atomic calendar made use of. That way, cancelled classes appeared with a strike-through on the calendar. I’m under the impression this cannot be done any more with the new CalendarEntity, but I’d be pretty happy to be proven wrong.
Is there a way to pass additional attributes for an event apart from the “normal” properties of a CalendarEvent ? Should I subclass it and if I do will any additional properties be serialized as JSON so frontend integrations can make use of them ?
Thanks for any help !