Hi Ján,
While working on scheduling support, would you be willing to take a look at also implementing support for consensus scheduling: http://tools.ietf.org/html/draft-york-vpoll? There should be a -01 draft out soon, but the original should give you an idea of what it entails. We are looking at doing a multi-vendor demo of VPOLL at our next CalConnect in May. Right now, the only client that we have is something that the Apple server guy mocked up in JS which talks to the Apple server. Since you have a fully functional CalDAV client, it would be awesome if CalDAVZap could also support VPOLL.
we can check it, but I cannot guarantee than we will have time to implement it (we are currently very busy) ... and things looks only worse, because Andrew Mcmillan has currently no time for DAViCal, so it looks like we need to fix a lot of things in it ourself :-/
That being said, I wouldn't be able to do any testing until I add support for VPOLL, which I also plan to do in the next month or so.