Please have a look at the attached (only lightly tested) patch.  I
didn't provide a similar service for `special-display-buffer-names' and
`special-display-regexps' yet.

`display-buffer' and `pop-to-buffer' behave "as usual" as long as the
default values for `pop-up-windows', NOT-THIS-WINDOW, and OTHER-WINDOW
are used.  An application calling `display-buffer' or `pop-to-buffer'
can specify the preferable window to split by setting NOT-THIS-WINDOW or
OTHER-WINDOW appropriately.  Users can specify their preferences (and
override the application) by setting `pop-up-windows' appropriately.

martin