> So...  there is no easy way implement this in the current Emacs?  Using
> post-command-hook and keeping track of previous cursor positions seems
> kinda ... non-optimal.

If you put the spacing display property on an overlay's before- or after-string, it doesn't create a new cursor position.   So would something like the below create the effect you're looking for?

(progn
  (setq o (make-overlay 1 2)
        o2 (make-overlay 2 3))
  (overlay-put o 'display (create-image "~/rms.jpg"))
  (overlay-put o2 'before-string (propertize " " 'display '(space :align-to (200))))
  (overlay-put o2 'display (create-image "~/rms.jpg")))