That is in fact how crash dump works on illumos systems. Once you enter a kernel panic, interrupts are disabled, all other CPUs are brought to rest, etc. Then the panic code takes the pages to write to the dump and passes them to a special routine in the driver for the dump device: usually either a slice on a disk or a zvol). That driver is responsible for getting the device into a state where polled I/O can be performed, and to write the pages out. At the end the system reboots, and on the next boot a program called savecore inspects the dump device to see if there is a new dump, and if there is it transfers it into a new file.