Having read ChrisForrence's comment I have done some experimenting and confirmed that the content of deleted posts can be viewed from the edit page. Furthermore, I'm able to successfully submit changes to deleted posts, and they're accepted without the need for approval (I don't have sufficient reputation to edit without approval on Stack Overflow, where I did the testing).
- Deleted Question (10k Only - kinda ;-) )
- Edit submitted by low-rep user, applied to the question above without approval
The posts timeline and revision history are not available, and after submitting an edit you are presented with a 404 page and it doesn't appear in your activity log. However I was awarded the Editor badge.
At my rep-level, I can't edit visible posts on Stack Overflow without approval (my edits go through the suggested edit review, and become visible on the post only when approved). I would expect at very least that the same restrictions would apply for deleted posts - but realistically, expecting folks to review edits on deleted posts sounds like a tremendous waste of time, therefore I should be prevented from submitting any edits to deleted posts with the exception of those I've written myself (which I don't need approval for when not deleted).
Furthermore, there's no good reason for me to ever see the edit form when I can't submit it - so attempting to load a URL such as this should result in a 404.
- Attempting to edit a locked or migrated deleted question does trigger an error: "This post is locked and cannot be edited."
- Attempting to edit a post on meta (where suggestions aren't allowed at all) triggers an incorrect error: "There is a pending suggested edit in the queue, try again in a few minutes." on non-deleted posts
- However attempting to edit a deleted post on meta still shows the editing page, though the edit is this time not allowed to be submitted1 (failing with a nice red box error).
1 Though the bug that allowed successful submission may have been patched in the meantime rather than this being a difference on metas.