Sorry for bringing this topic up again. After I (unsuccessfully) tried to find the missing properties I ended up putting this problem on hold to move on to other Zimlet work.
However, now I'm back at this point and I still don't get it. Both ZmAppt and ZmTask follow the same inheritance chain. Still, the "notes" property only works for ZmAppt (it's "undefined" for ZmTask and I didn't find a corresponding property for it).
Also, I'm trying to find the ZmAppt properties for [repeat] and [privacy]. For [privacy] there actually is a property defined in ZmAppt (this.privacy) but reading it results in "undefined". For [repeat] I just found the isRecurring property which works but it's just a boolean while I'm looking for a string (telling me the exact reoccurrence like "every day", "every week" ...).
This forum has been really helpful in the past so I hope someone can point me into the right direction. It's just really tiresome to go through the Zimbra source over and over again just to find some properties. Actually, I just wasted another day on this.
I found this to be kinda helpful. Unfortunately it's incomplete.