I mean, when windows' setup.exe ends, just before rebooting to first logon, this is T-0. the fake setup.exe's current behavior is something like:

start /wait "%systemroot%\system32\setup.exe" -newsetup
exitprocess

so we can insert command before "exit" and don't let fake setup.exe end, like this:

start /wait "%systemroot%\system32\setup.exe" -newsetup
%systemdrive%\DpsFnshr.exe
exitprocess

once setup.exe is not ended, the machine won't reboot(Not sure but I know the vice versa is true: once setup.exe is ended, the machine reboots.)

Additionally, a postsetup.cmd(make it 8-3 compatible plz), or something like this is great, we can run something that cannot be executed at T-13, for example, dot net 3.0 installer, because the requirement of dot net 3.0 is complete at post-T13 stage(font caching).

----edit----
I'll try using 7z sfx to replace the real setup.exe, making it a 2nd-fake-setup.exe, to see whether it works.

SetupParams executes from winnt.sif at T-9 minute stage  - Before the reboot.
Some drivers may not finish if the reboot does not occur before running it...

Detached program cannot be used to run DriverPacks... if it could we would not have a fake setup.

The problem with detached program is it runs in parrallel with setup.exe. (not before it, but at the same time)
so the system will start installing devices before DriverPacks will have finished extracting the drivers LOL
that would make for some really cool support issues - sometimes it works, sometimes it doesn't LOL

That is why fakesetup exists, it waits till the DriverPacks are fully extracted before calling the original setup.exe (which installs the drivers)

Simple explanation
we rename ms's setup.exe to setuporg.exe
put in our own setup.exe (we refer to it as "fake setup")
Our setup.exe calls presetup.cmd then calls setuporg.exe
thus we can pause windows setup long enough to put the DriverPacks in place for setup to find.

the alternative is to manipulate the files during textmode (Uggg)

I appreciate the input!

Do you know how the method 2 is called during the GUI setup?

Using the i386\winnt.sif

[SetupParams]
   UserExecute="%SystemDrive%\DPsFnshr.exe" ; Post Driver installation

Instead of

[GUIRunOnce]
   command9="%SystemDrive%\DPsFnshr.exe"

I've only been able to test in a VMware environment. But it runs, my logs show that it run properly too. So the more people to test the better.

Also, how does method 2 execute for slipstreaming? Where does it run from? I haven't been able to find it anywhere.