In emacs, is there a way to Hex edit/view a buffer without losing the undo info

Type C-u before either M-: or M-! to get the result inserted instead of sent to the echo area.

To get things directly into the kill ring, you need to dabble in Elisp. Something like this (untested):

;;; kill ring version of M-!
(defun shell-command-to-kill-ring (command)
    (interactive
      (list
        (read-shell-command "Shell command: " nil nil
                (let ((filename
                       (cond
                    (buffer-file-name)
                    ((eq major-mode 'dired-mode)
                     (dired-get-filename nil t)))))
                  (and filename (file-relative-name filename))))
        current-prefix-arg
        shell-command-default-error-buffer))
    (kill-new (shell-command-to-string command)))

;;; kill-ring version of M-:
(defun eval-expression-to-kill-ring ()
    (interactive)
    (call-interactively 'eval-expression)
    (kill-new (car values)))

First of all, in case you don't know about it, Emacs has hexl-find-file which opens up a file in hex editing mode. I know that it's not what you asked for, but if you're already using one, and you're comfortable with Emacs, then it's good to know about it for future needs.

Second, for this kind of "raw" editing of a file (which I tend to do often), find-file-literally is really great. It does what you'd expect it to do, and pretends to be a pre-unicode version of itself and open the file with escapes showing up for non-ascii characters (and control chars etc). This is likely to do what you want, though it does have the obvious disadvantage of not being able to actually read the text if you have a lot of non-ascii content.

So going down further into primitive support, there's the enable-multibyte-characters variable and the set-buffer-multibyte function that is used to toggle it. The nice thing about this is that it changes the buffer presentation dynamically -- for example, try this:

(defun my-multi-toggle ()
  (interactive)
  (set-buffer-multibyte (not enable-multibyte-characters)))
(global-set-key (kbd "C-~") 'my-multi-toggle)

and you now have a key that toggles the raw mode dynamically. It also has the nice property of leaving the cursor in the same place. But this raw mode shows you the internal representation (which looks like UTF-8) and not whatever the file happens to be using as its encoding. It should be possible to do what you're talking about with some hack (for example, using find-file-literally on an open file will ask you about revisiting it, but that resets the location and reloads the file too) -- but it sounds like the above is already fine. (That is, my guess is that you're trying to edit some text field in an otherwise binary file...)