Git-dit is, obviously, git specific and will hence not work with other VCS. Import and export functionality for other issue tracking systems are, however, not inconceivable. Functionality like referencing issues across repositories will, however, most likely never be provided across different VCS.
Git-commits are immutable. Hence, a message can not be edited once it is published. Commits/messages may, however, be "amended". This operation is equivalent to creating a new commit with the edited content and discarding the old one. Hence, if a reply already exists on the old commit, it will not be removed and the reply will still refer to the old commit. However, we do not consider this restriction sever, especially when compared to mailing-lists, which inhibit the same restriction. Note that we currently still lack a tool for convenient amending of messages.
Issues and related messages have to be pulled by the maintainer from public repositories. Otherwise, issue reporters would require push access to the bug repository. Originally, we planned (and still do plan) to provide a tool for automated imports of issues and patches from mailing lists. A bug repository may also be accessed via a web front-end at some point.
For another project, we also consider a notification mechanism for cross-platform notification of events in git repositories (e.g. pushes). Using such a mechanism, contributors could notify a project maintainer about changes in their public repositories, including issues or messages, via a public API. A maintainer could set up automated fetches of new issues, importing new issues and messages. However, this is unrelated and still work in progress. Additionally, it would still require reporters of issues to expose them via a public repository.
It was also suggested that maintainers or service providers could provide special public bug repositories with public push access. Those repositories would have specially crafted hooks installed, implementing some sort of access control, e.g. preventing issues from being deleted by unauthorized actors.