Defending mounted filesystems from the root user

I'll agree that it does seem theoretically possible to do so. Though I am doubtful that it is a good idea, besides the already mentioned concern of practical feasibility.

Imho either you trust your hardware, and don't want your FS drivers to be slowed down by being implemented super defensively, always rechecking everything etc. Or you don't trust, but then you should be fine taking the perf hit by using FUSE or a VM to isolate the hardware handling from your host kernel.

Having said that, I always dream about a new OS kernel that transcends the monolithic/micro-dichotomy by easily allowing to move all kinds of driver into userspace and back ;)