Found the solution:
Create a package with the following VBScript:
----------------------------------------------------------------------------------------------------------
set env = CreateObject("Microsoft.SMS.TSEnvironment")
Name = inputbox("Rename Computer" ,"Please enter the new Computer name below:",env("_SMSTSMachineName"),400,0)
env("OSDComputerName") = Name
-----------------------------------------------------------------------------------------------------------
Add a "Run command line" step to your Task Sequence, and run the script (put this step right after the "format and partition disk").
This will display pop-up box for you to enter the new computer name.
Hope this helps - works in my lab