The equivalent to flushing your register state is more comparable to a context switch than a complete shut down. Also Thanks to PCIe CPU swapping could be done in pure software at the OS level. No further hardware changes required. Simply copy the contents of your RAM to the other CPU card and manage the hand-off. This is clearly different from a complete shutdown e.g. all devices on the PCIe bus would be able to keep their state.

A good programmer familiar with the target OS and inner workings of PCIe could deliver a working prototype within a week, production ready within less than a year.