As discussed on the main list, here's the incremental autorebase package
as well as the patched setup.exe to implement perpetual postinstall
scripts.
$cygwin=http://cygwin.stromeko.net/
wget $cygwin/noarch/release/_incautorebase/_incautorebase-00006-1.tar.bz2
wget $cygwin/x86/setup-x86.exe
wget $cygwin/x86_64/setup-x86_64.exe
As briefly mentioned, the implementation was done to minimize the
changes to both the infrastructure and the size of the patch to setup.
This could be implemented differently, for instance based on the
directory the script is in, rather than its name. Also, in the same way
one could look at the name of the script and have certain scripts run
before or after the others, or use hidden groups (like _PostInstallLast
and have it actually do what that name suggests).
Another obvious extension would be triggers, i.e. the installation of
one (or multiple) packages will result in the execution of the triggered
postinstall script. This is useful if the same postinstall action can
be used by many packages (texlive would benefit enormously from that).
This would require changes to the packaging, upset, setup or all of
them, of course.
I suggest to start with this and take it from there.