Explorer.exe should be running in the context as the user you are looking for. Please try this and see if works for you. I would appreciate you replying with your results.

? GetProcessOwner("explorer.exe")
 

Function GetProcessOwner($Exename, optional $remotepc)
  dim $sc,$processes,$process
  if $remotepc=""
    $remotepc="."
  endif
  $sc = CreateObject("ScriptControl")
  $sc.language = "VBScript"
  $Processes = GetObject("winmgmts:{impersonationLevel=impersonate}!\\" + $remotepc + 
                         "\root\cimv2").ExecQuery("Select * from Win32_Process where Name='" + $exename + "'")
  For Each $Process in $Processes
    $sc.addobject('vbProcess',$Process)
    $sc.addcode('Return = VbProcess.GetOwner(strNameOfUser)')
    $sc.run
    $GetProcessOwner=$sc.codeobject.strNameofUser
  next    
Endfunction