Well, I have to take the undocumented statement back ...
It is a Method of the System object, so I didn't find it immediately :
Kixforms Class Library 
Shell Method  
Description
Loads and runs a program. 
Syntax
object.Shell(command, window-style, redirection) 
Parameters
command 
Command can be any 16-bit or 32-bit application. To run command interpreter commands, specify the correct command interpreter as part of the command. 
window-style (optional) 
One of the following values: 
Value Setting Description 
0 Hidden Window is hidden and focus is passed to the hidden window. 
1 Normal (focused) Window has focus and is restored to its original size and position. 
2 Minimized (focused) Window is displayed as an icon with focus. This is the default. 
3 Maximized Window is maximized with focus. 
4 Normal Window is restored to its most recent size and position. The currently active window remains active. 
6 Minimized Window is displayed as an icon. The currently active window remains active. 
redirection (optional) 
One of the following values: Value Setting Description 
0 None No redirection. This is the default. 
1 Stdout Redirect standard output. 
2 Stderr Redirect standard error. 
3 Both Redirect both standard output and standard error. 
Return Value
If redirection is specified, returns the redirected output as a string; otherwise, returns the integer exit code of the command. 
Remarks
Script execution is stopped until the program exits. 
Example
$Listing = $System.Shell("cmd /c dir c:\myfolder", 0, 3)
For Each $Line In Split($Listing, @CRLF)
 ?"Line=" $Line
Next
See Also
Applies To: System 
It seems to have problems taking %ComSpec% as input though ...