It sounds like some of the PCI controller docs for chips
used in various SPARC platforms have now been made available at:
http://www.sun.com/processors/documentation.html
I haven't gone through to see which platforms they cover and
if they fill in any missing gaps in Xorg's PCI bus code, but
someone who knows more than I about PCI (which is most everyone)
is welcome to find out.
(Don't ask me what other docs are coming - I just found out about
these from http://www.jmcp.homeunix.com/roller/jmcp/entry/200708161 )