This won't work for all filesystems because they often increase a mount count even if mounting read-only, thus rendering it impossible to verify hashes.
It should be possible of course with software, to mount a writable media read-only, in such a fashion that NO writes are done whatsoever to it.
That's a nice theory, and honestly, I would think that's the case with the linux kernel. However, consider that a lot of forensics are done using Windows boxes. It may be a crutch in theory, but in practice it's apparently the difference between having your evidence thrown out and having it admitted.
If that is not possible, then it should be possible to use a software modification which causes the block device driver to behave as a readonly one, even if the device is writable.