The next logical step to using Task Sequence Variables for existing systems is to use them for unknown computers. For that you need a user dialog box to appear at the start of the Task Sequence to allow the person building the new PC to select what options to include in the build. You might want to be very specific about the drivers you install, for example.
For a Task Sequence with a 32bit boot image Psexec.exe is perfect for launching a dialog in Session 0. Without the option to do this the dialog does not appear. The application will run but it cannot display a GUI without having been launched in the same session at the Task Sequence.
However, 32bit is a bit old in the tooth. Most sensible people are now switching to 64bit operating systems. Psexec.exe however is a purely 32bit application that will not run on a 64bit WinPE image because WinPE does not support Windows on Windows (WOW64.) There are instructions on Technet Blogs about how to write your own application, which you then have to go search out each step of the way.
I did this and found the path a bit harrowing. It's not for the feint hearted but it's doable. I managed to write it first time. To make the application specific to Task Sequences (and not exploitable for other uses) it is written in such a way that it is tied to the task sequence process and will otherwise fail. If you like you can download it from here.
To use it, add it and the dialog executable to a package. In the task sequence use a "Run Command Line" option. Specify the package and for the command-line use tslaunch.exe
This was built for .Net 4.5.1 and works with the Window 8.1 64bit WinPE image (6.3.9600.16384)
If you would like the source code, please contact me on Google+ and I'd be happy to oblige.