Well, it seems I spoke too soon. As the Mozilla folks have been doing an awful lot recently, they have come through. Sunbird (and the Mozilla Calendar extension) now do basically what I want and I am the happiest boy alive.
It’s still a little hokey and there are some bugs, but I can definitly manage a calendar on my PCs and my Mac, all accessing the same file on a central server. I’m using WebDAV to do it. You have to jump through a couple of hoops to get it to sync up properly the first time, but after that it’s perfect.