Lots of use cases - Rolling kernel upgrade

I'm not sure having multiple kernels accessing the same block device/filesystem is going to work very well. If it were going to work at all you'd probably need a separate virtual filesystem driver for the new kernel, which talks to a server on the old, and then once the old kernel is ready to unmount the filesystem do a switcheroo-handover type thing. The new kernel would have to swap out the virtual filesystem driver for the real ext4/btrfs/... and start accessing the "real" inodes directly?