Let see:
-- per-issue/component authorization (secrecy!):
git-bug already support internally to attach crypto keypair to an identity and sign/verify the created commits. What's missing is proper tooling around that, ACLs and a way to configure them (a project settings datastructure), possibly a way to encrypt entirely the stored data. It's planned for the most part, good to have otherwise.
-- ease of access to non-git users:
the goal is to have the webui to act as the public portal with external auth (github or your company SSO). I definitely agree that a bugtracker should not be just on the dev computer and should not require special tooling.
-- history rewriting/sanitizing:
not so sure what you really want here. Editing comments is already possible. Otherwise, it's still possible to just delete a bug or add more concepts in the data model (archive, delete comment ...).
-- built-in large attachment hosting:
Definitely planned. The data model already support that, it's just a matter of exposing it in the different user interfaces. Now for the storage usage, see below.
-- gigabytes of issue data and generally large scale deployments for large organizations:
The funny thing when you put all the data in git is that the problem becomes "how to host git at scale", which is a sort of solved problem already. On top of the obvious, there is solutions like git LFS if you don't want to retrieve all the potentially large attachments. Also, as each bug is independent from each other, you can also shard a repo or just retrieve the bugs you care about with some filter.