AppScript is based on VB 4.0, so there may be a vbscript 4.0 function that can be used.  There is a limit of the type of objects that can be created in AppScript.

You can use something like Ext.NewTask or Ext.NewTaskWait or Ext.CmdLine to call out to a command window where you can issue a Powershell command that will run a ps script that performs your REST or SOAP Call.  The trick here will get where to store the results of the call if you need to continue processing the returned data.

There is an option to create your own C++ dll that can be loaded into an AppScript to call a given function.

ModScript does have this ability to make REST calls and then process the data returned.  See REST Callouts.