I have a couple of patches that add ACLs to keyrings, that you can find at the
top of the branch here:

	https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=keys-acl

I have other patches that allow tags to be used as subjects in the ACL, with a
container supplying a tag, e.g.:

	https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/commit/?h=container&id=e0fdc5613a32b9475b025f58e7a2267329b3f3a4

Might something similar be of use to you here, perhaps specifying a tag
referring to the blockdev of interest rather than the container?

David