Look at drivers/char/ipmi/ipmi_msghandler.c. It has code to send panic event over IMPI. The code is pretty complex. Of course if we a going to implement something more complex than simple hypercall for panic notification we better do something more interesting with it than just saying "panic happened", like sending stack traces on all cpus for instance.
I doubt that's the best example, unfortunately. The IPMI event log has limited space and it has to be send a little piece at a time since each log entry is 14 bytes. It just prints the panic string, nothing else. Not that it isn't useful, it has saved my butt before.
You have lots of interesting options with paravirtualization. You could, for instance, create a console driver that delivered all console output efficiently through a hypercall. That would be really easy. Or, as you mention, a custom way to deliver panic information. Collecting information like stack traces would be harder to accomplish, as I don't think there is currently a way to get it except by sending it to printk.
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