I have submitted PR# port-sparc/15963 for 24-bit support in the tcx
driver; this works with the S24 card that plugs into the AFX slot on
the SS5.  The patch included in this PR provides cg8 emulation as a
compile-time option, similar to that provided for the cg14 (and with
the similar limitations: no hardware acceleration, no special handling
for console output while the framebuffer is in 24-bit mode, and the X
server must be modified to use 24-bit TrueColor mode rather than its
preferred mode of 24-bit DirectColor).

The driver diffs, description of changes, and instructions for patching
Xsun24 to use 24-bit TrueColor are available in the PR at
<http://www.NetBSD.org/cgi-bin/query-pr-single.pl?number=15963>

Charles

p.s.  Thanks to Jeff for running the tests and describing the results
before I got a S24 of my own...