Can the notion of "display" (X) approximate each monitor? In a
situation with two monitors, one could have x-display-list return
three displays: a main one ("Mac"), and then "1" and "2" or so for
the other two, separately. Then, x-display-mm/pixel-width/height
could either return the dimensions of the total area, or of only one
of the screens.
According to the manual page of X, "the phrase `display' is usually
used to refer to collection of monitors that share a common keyboard
and pointer (mouse, tablet, etc.)." In that sense, there's only one
"display" on a Mac regardless of the number of monitors.
"Display" and "screen" are established terms in X11, and I think we
should not abuse them. As I mentioned earlier, the concept you want
has another name "framebuffer" in the X11 (Xinerama) world, and I
think the right thing is to support them in a consistent way on the
relevant platforms.