So, after spending the better part of the day working on this, I now have an #iCal file that contains _one_ event for Easter, which is using an RDATE property to list all of the Easter dates in about 200 years (because it's an irregular event and normal repeat rules won't work).

Thunderbird accepts this (even if it says "repeat details unknown"), #khal on the command line does, too.

@mailbox_org's #OpenXchange web UI doesn't display them.