Integrating .NET Framework (any version) into Windows PE (WinPE) is a bit challenging. Running it, however is not...

The solution to get support for tools like PowerShell in Windows PE is to have .NET Framework running. This can be solved by virtualizing .NET Framework and the .NET application you want to run.

I have successfully used Thinapp (beta) from Vmware to run PowerShell (and other .NET apps) in WinPE:

Tip!
Since the systemdrive in WinPE is X:, make sure to select the WriteCopy isolation mode when capturing .NET Framework (or any other app) for use in WinPE.

Try the Thinapp beta at:
http://communities.vmware.com/community/beta/northstar