>> X11 shouldn't really have a problem with [unusual colour depths]
> The X11 protocol does impose limits on the bpp of pixmaps -- 1, 8,
> 16, 24 and 32, IIRC.

4 as well - and a surprisingly large fraction of the code Just Works at
2bpp even though it's not part of the spec.  (Vide infra.)

But that list is just for pixmaps on the wire.  There is no reason you
couldn't have a 13bpp display with pixmaps padded to 16bpp on the wire;
the wire encoding depth is only loosely related to the actual depth.

> For example, NeXT hardware has depth 2 at 2 bpp, which is exported to
> clients as depth 2 at 8 bpp,

At 4bpp, I believe - or even 2bpp; when I did X for black NeXT hardware
(back in the X11R4 days) was when I found that 2bpp worked surprisingly
well even though it's formally out of spec.

> Similarly, X servers for plain VGA cards use depth 4 at 8 bpp.

I think they generally use 4bpp on the wire.  At least in R4, 4 is one
of the permitted bits-per-pixel values for a pixmap-format as sent by
the server during connection setup (I just checked).

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